Forward-reverse drive control providing automatic braking
United States Shore atem [1 1 [4 1 Apr. 8, 1975 [75] Inventor: Daniel B. Shore, Niles, 111.  
 [73] Assignee: International Harvester Company,  
 Chicago, 111.  
 [22] Filed: June 6, 1974 [21] Appl. No.: 477,129  
  Related U.S. Application Data [60] Division of Ser. No. 149,853, June 3, 1971. Pat. No. 3.843.205. which is a division of Ser. No. 830.035. June 3. 1969, Pat. No. 3.606.906. which is a continuation-in-part of Ser. No. 689,549. Dec. 11, 1967. Pat. No. 3.458.018.  
 [52] U.S. Cl ISO/103; 180/105 [51] Int. Cl 360k 27/04 [58] Field of Search 180/82 A, 103, 105; 303/];  
 Primary E.\&#39;aminerDavid Schonberg- Assismnt E.\&#39;aminerTerrance L. Simmons Attorney, Agent, or Firm-John W. Gaines; Floyd B. Harman [57] ABSTRACT Tractor having dump valve and forward-reverse drive control employing same, effective to automatically brake the tractor when reversing the tractors direction of motion. The tractor has brakes, a reversible transmission controlled by the dump valve. a range selector valve having a control portion, and a run detector pump to sense direction of rolling movement of the tractor and having a directionalized pump output. The dump valve generally receives, against pressure movable end areas thereof, an output from the range selector valve and directs control pressure to operate the brakes and transmission in alternation to one another and, specifically: receives against the pressure movable end areas the directionalized pump output so as to be hydraulically balanced in a centered position operating the tractor with transmission engaged and brakes disengaged; receives against the pressure movable end areas an unbalancing force from the range selector valve control portion causing the dump valve to shift off center and dump, neutralizing the transmission and setting the brakes; and having valve springs effective to mechanically rebalance the dump valve attendant with slowdown and drop of unbalancing pressure from the run detector pump, causing the dump valve to recenter, releasing the brakes and engaging the transmission in an opposite drive setting.  
 4 Claims, 4 Drawing Figures PAT NTED APR 81975 SHEET 3 0f 3 FORWARD-REVERSE DRIVE CONTROL PROVIDING AUTOMATIC BRAKING This application is a division of Ser. No. l49853 filed June 3, 1971, now US. Pat. No. 3843205; which is a division of Ser. No. 830035 filed June 3, 1969, now US. Pat. No. 3606906; which is a continuation-in-part of Ser. No. 689549 filed Dec. 11, 1967, now US. Pat. No. 3458018.  
  The present invention relates to a vehicle provided with conventional brakes, and to an interacting vehicle transmission provided therein and operated by a forward-and-rear drive control which is provided so that, when being reversed, the transmission at once neutralizes while the brakes are progressively being applied automatically, and subsequently the transmission goes into the opposite or reverse drive setting as and when the brakes have brought the still rolling vehicle to the point at or near standstill, at which point the brakes release automatically without delay.  
  In the foregoing manner, my invention provides for conditioning of the operating mechanism in the vehicle so that as each reverse shift is being completed, the takeover shock encountered by the reverse power path through the transmission is materially reduced or substantially eliminated. While the invention is primarily applicable to loaders and industrial construction and agricultural tractors, it has equal application to trucks and other vehicles equipped with a transmission drive, particularly a power shift transmission.  
  More specifically, I provide pressure-signalresponsive work performing means for applying power to engage and disengage the brakes and to disengage and re-engage the transmission, and a pressure-signalproducing run detector pump responsive to vehicle speed connected to the work performing means to reengage the transmission and to disengage the brakes when, during a braking phase, the vehicle is substantially at no-run.  
  Shuttle work to-and-fro constitutes such a major part of the utility of industrial construction, agricultural, loader, and like vehicles, that a suitable power shift transmission or hydrostatic transmission is desirable to expedite the speed changes and direction changes that are a frequent necessity to the operator. Problems arise when shuttling many such vehicles because reverse shifting is tedious and time consuming, and very often puts the transmission under undue strain due to shock loading of the reverse power path in effecting the change of direction.  
  My invention alleviates the foregoing problems. It especially alleviates the situation arising, first, in the forced reverse shift situation encountered with a power shift transmission and, second, arising in such a short interval, and without conditioning the operating components of the vehicle, that the strain is excessive, as will now be explained.  
  Features. objects, and advantages will either be specifically pointed out or become apparent when, for a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawings, which show a preferred embodiment thereof and in which:  
  FIGS. 1 and 2 are right side and rear end elevational views to small scale and large scale, respectively, of a tractor embodying the present invention;  
 FIG. 3 is a top plan view of a detail of FIG. 1; and  
  FIG. 4 is a hydraulic circuit diagram, illustrated with single lines, of the tractor control system.  
  More particularly in FIG. 1 of the drawings, the rear portion of the illustrative vehicle 10 which is shown is part of a crawler tractor for use in moving, loading, or carrying earth or other material. A longitudinally extending tractor frame 12 carries a body 14 of the vehicle and is carried by left and right longitudinal endless tracks 16 and 18 disposed one on each lateral side of the frame 12. The vehicle 10 proceeds in a straight line so long as the tracks 16 and 18 are driven at the same speed and the vehicle is steered when the speed of the respective tracks differs.  
  The interacting components providing the essential control parameters according to my invention comprise first, a range selector valve 20 forming part of a transmission range selector control tower A and second, a run detector comprising a transmission signal pump 22 which is connected to the sprocket drive pinion shaft 24 forming part of a sprocket final drive system B. The two components 20 and 22 interact in a varying manner to control a neutralizable transmission system F which drives the vehicle and an automatic power brake cylinder 26 which is connected to a brake linkage 28 forming part of the vehicle braking system G.  
  By the term neutralizable transmission system F, what I mean broadly is any hydrostatic, hydromechanical, or mechanical drive, or a power-shift, friction drive such as includes at least one opposite brake or clutch which remains inactive to reverse the drive until a companion operative brake or clutc&#39;n has definitely been declutched, all having an effectively de-coupled condition of receiving input and yet transmitting no power output.  
 BRAKES: MECHANICAL CONNECTIONS-FIGS. 1 AND 2 The brake pedal linkage 28 is actuated by being either drawn forwardly by a brake pedal, not shown, or pushed forwardly by the automatic power brake cylin der 26 to set the brakes of the vehicle braking system G. At the rear, the brake pedal linkage 28 is connected to a brake applying arm 30 which is pulled forwardly when the linkage is actuated. The arm 30 operates in a mechanical path by means of an outwardly moving, push-rod connection 32 and a longitudinal arm 34 of a pivot brake bellcrank 36 to apply a caliper brake against a brake disk 38 in the right sprocket final drive system B to the track 16. Similarly, the arm 30 operates by means of another outwardly moving, push-rod 40 to apply a caliper brake against a brake disk, not shown, connected in the left sprocket final drive system to the track 18. The linkages on both sides are the same, and operate the two caliper brakes simultaneously.  
  More specifically, the push-rod 32 and bellcrank arm 34 connection pushes outwardly against the upper end 42 of a caliper lever which is pivoted at its lower end on a fixed pivot 44. A shoe, not shown, which is carried by a pivot 46 on the caliper lever is forced against the adjacent brake disk 38 and the reaction is taken by the main case 48 of the final drive.  
 BRAKES: HYDRAULIC CONNECTIONSFIGS. 1 AND 2 The brake pedal linkage 28 receives hydraulic assistance from two, transversely aligned pivot cylinders 50,  
 the linkage 28 controlling the cylinders through valves 52 in communication with the respective cylinders and carrying the respective valve operating levers or arms 54. The linkage 28 has a single connection to the two arms 54 jointly so as to operate the cylinders 50 in unison.  
  As illustrated in FIG. 2, each pivot cylinder bellcrank 36 has a lateral arm 56 in engagement with a pivot cylinder 50. The arms pivot the bellcranks 36 when two cylinders 50 extend, causing engagement of the vehicle braking system G at both sides of the vehicle.  
 STEERING DRIVE SYSTEM-FIGS. 1 AND 2 Each of the pivot cylinders 50 is part of a series of three hydraulic boosters, each series being disposed at one side of the vehicle for controlling the sprocket drive pinion shaft 24 at that side to steer the vehicle. The valving for the boosters is included among the valves 52, and the arms 54 operate all such valves.  
  A steering drive system H includes right and left steering levers S6 and 58 which are slightly forward of the operators seat 60 and which are operated by hand so as to swing fore-and-aft in vertical planes. Each steering lever has a separate connection, not shown, to an arm 54 for operating the steering valving independently. The steering system H also includes each of the brake disks 38 (FIG. 2).  
 FINAL DRIVE B-FIG. 2  
  Each of the sprocket drive pinion shafts&#39;24 has one of the brake disks 38 fast thereto, and also a pinion 62 fast thereto. The two pinion shafts 24 are jointly driven by a reversible shaft 64 which the power shift transmission system E drives, and one of the pinion shafts 24 drives the run detector pump 22 by means of intervening gearing 66.  
  The pinion 62 on each shaft 24 meshes with a bull gear 68 in the final drive B at each side of the vehicle. Depending upon the gear reduction desired, each bull gear 68 can directly drive the drive sprocket of the adjacent track or can be connected to the sprocket by reduction planetary gearing, not shown.  
 RANGE SELECTOR VALVEFIG. 3  
  The body of the range selector valve carries a range spool 70 mounted for rotation and reciprocation therein. A selector lever 72 has a pivot connection at its midportion to the forward end of the range spool 70 and causes the spool 70 to be set in appropriate positions as the lever 72 is moved by the operator among a neutral position indicated at N in solid lines, a forward neutral position indicated at FN in broken lines, a reverse neutral position indicated at RN in broken lines, and respective dirve positions F1, F2, R1, and R2 which are opposite similar markings on a slotted gate plate 74 mounted atop the transmission range selector control tower A. The slot 76 in the plate 74 receives and guides the lever 72 and is of a general U shape.  
  The body of the range selector valve 20 has, in addition to the drain lines indicated at D hereinafter, a set of oil lines connected thereto including lines F2 and R2 as shown, and N, FN, RN, F1, and R1, not shown, the abbreviations being functional drive designations including neutral (N).  
 CONTROL CIRCUITFIGL 4 Regular valving, not shown, provides main, lube or intermediate, and drain pressure at the respective points abbreviated by their functional designations M, L., and D, previously referred to. For simplification, all points and lines designated D can be maintained at the lube pressure L if desired.  
  The body of the range selector valve 20 is drawn so that it appears at two pointss in FIG. 4, the range spool according to this manner of illustration being split out and shown in both a longitudinal view 20a and a transverse sectional view 20b for ease in understanding. The range selector valve 20 is connected via aset of transmission valves 78 with directional clutch packs in the transmission system E, specifically, forward and reverse speed packs 80 and 82. The already enumerated set of oil lines interconnects the selector valve 20 and the transmission valves 78, and four service lines 84, 86, 88, and interconnect inlet-outlet ports in the respective transmission valves 78 and the clutch packs 80 and 82 to provide forward and reverse transmission speed settings. Two shafts F1 and R1 extend to establish the low speed forward and reverse drives in the transmission system E, and two shafts F2 and R2 extend to establish high speed forwad and reverse drives, respectively. The clutch packs are conventional, neutralizing the existing drive setting without delay and progressively engaging a drive as selected.  
  In brief, the range selector valve 20, which is a multiposition valve, and the run detector pump 22,which is a reversible pump, produce directionalized pressure signals by which they mutually operate a comparatordump valve spool CD among the transmission valves 78 to coordinate neutralization of the clutch packs 80 and 82 and transmission, and application of the automatic power brake cylinder 26. In addition to having such coordinated action of neutralization and brake application, the transmission system E and vehicle braking system G also have independent operation for the usual purposes.  
 INDEPENDENT TRANSMISSION NEUTRALIZATIONFIG. 4  
  In the neutral setting of the range selector valve 20 wherein the range spool has the position 200 as shown in its longitudinal view and the position 20b as shown in its transverse sectional view, all in solid lines, the four oil lines F1, F2, R1, and R2 are connected in common to, and are at the pressure of, the drain line D.  
 The neutral line N on the valve 20 and the main pressure line M on the valve are interconnected thereby,  
 and a double check valve 92 leading to the power brake cylinder 26 unseats in a rightward direction such that the oil line N communicates, and sustains main line pressure in, the rod end of the power brake cylinder 26. The force of the pressure in a rod-end chamber 94, coupled with the force of a brake release spring 96 in the chamber, overcomes the force of main line pressure M constantly maintained in the head end chamber 98 of the cylinder 26. Any fluid trapped in the head end is thereby forced through a check valve 100 which unseats to allow the fluid to enter therethrough into the main line M. The brake cylinder 26 is held in the foreshortened, brake release position.  
 Irrespective of whether the run detector pump 22 isstopped or being operated due to coasting of the vehicle in one direction or the other, there is no further pressure in an active circuit in the hydraulic control and in such condition the transmission system E remains in neutral and the brake system G remains disengaged.  
  A similar condition obtains with the vehicle either stationary or coasting, when the lever is moved into one of its additional transmission neutral positions RN or FN.  
  When the vehicle is coasting in either direction, e.g., forwardly, and the range spool is rotated clockwise from the position shown in solid lines 2012 so as to take a position to the right corresponding to that same forward direction of the vehicle, no further pressure occurs in an active circuit in the hydraulic control. Coasting of the vehicle in that direction, e.g., forward, when the range spool is rotated leftwardly from the position shown in the solid lines b into the counterclockwise position, causes fluid to flow indirectly from the F side of the run detector pump 22, through a line J, the valve 20b, a line R, and thence from one direction into a double check valve 102 in the line R and against a pressure movable end area 104 carried by the comparator-dump valve CD and physically located in a pressure operating chamber 106 at that end. The run detector pump also directly applies pressure from a different direction into the double check valve 102 and against the same pressure movable area 104 carried by the comparatordump valve CD. An opposite pressure movable end area 124 on the valve spool CD is meantime connected by a double check valve 136 to the drain side of valve 20 and to the suction side of the pump 22. The spool CD perforce shifts to the right as viewed in FIG. 4, causing only the right side of the double check valve 92 to be relieved of pressure in a path from the valve 92, through an oil line 108, a gallery-shaped common transfer chamber 110 in the set of transmission valves 78, a spool undercut 112, and thence through a cored cavity 114 into a gallery shaped common lube pressure chamber 116.  
  So while the transmission has not changed setting, the set of valves 78 has been pre-conditioned for a change due to shift of the spool CD, and the double check valve 92 has been pre-conditioned to change the brake condition because only one side thereof (connected to oil line N) remains pressurized.  
 NEUTRALIZATION WITH AUTOMATIC BRAKINGFIG. 4  
  This situation is made to occur when the preconditioning just described exists, i.e., the vehicle is coasting in one direciton, e.g., forward, when the range spool has reached the transition position (RN) corresponding to the opposite vehicle direction. From the latter position, the spool from the position corresponding to the longitudinal view 20a is shifted rightwardly, as seen in that view, into the R1 position also known as low reverse. The only pressurized oil line N connected to the double check valve 92 is vented to drain D through the range selector valve 20, relieving pressure in the rod end chamber 94 of the automatic power brake cylinder 26.  
  The brake system G is applied because the hydraulically unopposed main pressure in the head end chamber 98 extends the cylinder 26 and, at the same time, the oil line Rl applies main line pressure to a pressure movable right end area 118 carried by the valve R2R1 and located in a pressure operating chamber 120. The valve R2Rl moves leftwardly as viewed in FIG. 4, forming a drain path in which the fluid can flow from the right side of the clutch pack 82, through the line 90, and thence through an undercut 130 to transfer chamber which is maintained at the lube pressure level. Hence, transmission system E remains in neutral during the deceleration period while the rolling vehicle is being braked by the braking system G.  
  At some point during the terminal portion of deceleration of the vehicle, speed of the run detector pump 22 decreases until the pump output pressure in the valve end chamber 106 is overcome by the force of a spring 122 surrounding the pressure movable end area 124 presented by the comparator-dump valve spool CD in a pressure chamber receiving the right end of the spool CD. The spool CD mechanically recenters, between the spring 122 and an opposite spring 138 balanced thereby, to its neutral or satisfied position, blocking the cored cavity 114 which is at lube oil pressure and communicating main line pressure to the right end chamber of the reverse clutch pack 82 through a path leading from a cored cavity 126 in the transmission valving 78 which is at main line pressure, through an undercut 128 in the spool CD, the transfer chamber 110, and an undercut 130 in the leftwardly shifted transmission valve spool R2Rl, thence through the service line 90 into the directional clutch pack 82 to establish the R1 drive setting. The clutch packs are of well known commercial construction, providing a quick, low pressure slack take up prior to application of full clutch engaging pressure, to minimize shock of engagement.  
  Simultaneously, the centered valve spool CD causes main line pressure to be applied to the automatic power brake cylinder 26 in a path leading from the cored cavity 126, through the undercut 128, the transfer chamber 110, and the oil line 108, thence through the right side of the double check valve 92 and into the rod end chamber 94 of the cylinder 26 to retract same and release the brake system G.  
  The vehicle then accelerates to the low speed in the desired opposite direction, i.e., R, and perforce the run detector pump 22 reverses direction so that oil flows therefrom in the direction of arrow R in a path leading through a check valve 132, a valve and restriction type, maximum pressure regulating circuit 134, the oil line J, the range spool 20b which occupies a leftward position from the position 20b shown in the solid line sectional view, and the oil line R, thence through the upper half of the double check valve 102 and into the pressure chamber 106.  
  Simultaneously, oil flows in the direction of the arrow R from the run detector pump 22 through the bottom half of a double check valve 136 to pressurize the pressure movable end area 124 on the valve spool CD.  
  As a result, the comparator-dump spool CD is hydraulically balanced and also mechanically balanced by the previously described action of the springs 122 and 138, each engaging one end of the spool opposite from the other. This hydraulic and mechanical balance of the spool CD in its satisfied position continues as the reverse speed of the vehicle increases, and will continue to be maintained if the selector valve is advanced into the high speed reverse (R2) position for the purpose of shifting the transmission into high speed reverse drive.  
 CONVENTIONAL DRIVE CHANGES-FIG. 4  
  When proceeding rearwardly, the transmission system E is readily shifted for the last mentioned purpose etween R1 and R2 positions by reciprocatory moverent of the range selector valve 20. The R1 condition as been described, and the R2 condition is attended y main line pressure M being applied by the selector alve through the oil line R2 into a pressure chamer 140. A pressure movable left end area 142 on the 300i R2Rl is depressed under pressure, shifting the 3001 R2R l to the right from the solid line position as iewed in FIG. 4. Main line pressure is thereupon di- :cted by the valve R2R1 in a path leading from the ored cavity 126, through the undercut 128, the trans- :r chamber 110, and a longitudinal central passage 44 in the spool R2Rl which is cannelured, a set of raial passages 146, valving 78, thence through the serice line 88 into the left chamber of the directional lutch pack 82 so as to cause high speed reverse drive r the transmission system E.  
  Changes in the forward drive range occur in convenonal manner as result of changing the range selector alve 20 between the F1 or F2 positions, thereby applyrg main line pressure in a pressure chamber 148 or 50 so as to shift the forward valve FlF2 respectively r the right or to the left as viewed in FIG. 4. Main line ressure thus enters through the seervice line 84 into re left chamber or through the service line 86 into the ght chamber of the directional clutch pack 80, the ther service line and chamber being connected at the me time to the common lube pressure chamber 116 r the housing of the valves 78.  
  In either of its directions of rotation, the run detector ump 22 always draws from one oil reservoir 152. One fa pair of seat-to-seat connected check valves 154 unrats to connect the reservoir 152 to the suction side of re pump 22, whereas the other one of the check valves rats to block communication and stop escape of the umped fluid from the output side of pump 22 back to re reservoir 152.  
  The check valve 132 unseats in the direction of a &#39;mmetrically disposed, alternately operable check rlve 156, which valves are connected to the output de of the pump 22. When either valve is unseated to low pump output fluid to flow into the regulator cirrit 134, the other valve stays seated to prevent loss of re regulated pressure back to the suction side of the Jmp 22.  
 The regulator circuit 134 provides a continual rericted bleed to drain D from the oil line I and, in addion, prevents the pressure from exeeding a predeterined value irrespective of the speed of the run detec- Ir pump 22. At the lower pump speeds, the output .115 below a predetermined reduced value, such that rring pressure of either spring 138 or 122 readily over- )mes the back pressure created by the circuit 134 and :centers the spool CD.  
 A restriction 158 which is connected between main 1e pressure M and the head end chamber 98 of the )wer brake cylinder 26 insures freedom from abrupt- :ss with a gradual application of the brake system G prevent shock. The restriction 158 is complemented the check valve 100 hydraulically in parallel thereith, so that the two form a flow-check valve combinaan. On the other hand, the check valve 100 unseats rd bypasses the restriction 158 during the reverse flow rusing brake release, so that the brake system G can sengage immediately to expedite vehicle direction ranges.  
  The forward clutch pack has individual restrictions 160 symmetrically connected to the opposite actuating chambers therein so as to provide for a residual oil pressure for lubrication purposes in the chambers at all times. Similarly, the reverse clutch pack 82 has analogously symmetrically connected restrictions 162 for lubrication purposes, such restrictions all discharging to drain D.  
  The complete schematic circuit of the present hydraulic control, except for certain novel additions hereof, appears in the same assignees copending Ruhl US. Pat. No. 3,386,523, the disclosure of which is incorporated in entirety herein by reference.  
 SUMMARY In summary, novelty is felt to reside in the beneficial results created by the comparator dump valve CD. To illustrate the salient points, let us say that the vehicle 10 is rolling rearwardly. The valve spool CD has three phases of operation while the vehicle is so rolling. In the solid line position shown in FIG. 4, the valve CD in a first phase directs high pressure fluid from the chambers 126 and 110 to the brake cylinder 26 to sustain the cylinder in the foreshortened, brake release position; the valve CD also directs fluid from the chamber 110 through the appropriate transmission valve and into the active end of the selected one 82 of the directional clutch packs to keep the transmission engaged&#39;and sustain rolling of the vehicle rearwardly and under power.  
  In a second phase, the valve CD has unbalanced position displaced to the right or to the left from the solid line position shown in FIG. 4, actually to the left under the assumed rearward rolling condition. Such position of the valve CD is attendant with movement of the valve 20 through neutral to a selected forward driving position, whereupon the valve CD dumps the pressure of transfer chamber 110 through the low pressure chamber 14 doing two things. The brakes progressively engage due to the valve CD dumping pressure from the rod end of the brake cylinder 26 which takes its extended, brake-apply position; and the driving clutch pack 82 releases the rearwardly driving clutch therein because the clutch cylinder activating pressure is dumped through the service line 88 or 90 leading to the transfer chamber which is likewise being dumped by the valve CD. The brakes force the coating vehicle to decelerate.  
 In the third phase, the valve CD recenters into the balanced position shown in solid lines in FIG. 4, re-  
 storing high pressure to the transfer chamber 110. Con-- sequently the brakes cylinder 26 goes into the foreshortened brake release position as attained in the first phase above, but a selected clutch in the forward clutch pack 80 is activated under the high, engaging pressure to engage the transmission in forward drive and change the vehicle from the former rearwardly rolling direction to the forwardly rolling direction desired.  
  The first phase of operation of the valve spool CD described results when control signals from the respective valve 20 and pump 22 hold the spool CD hydraulically in balance from the respective equal and opposite pressure forces on the opposed pressure movable valve areas 104 and 124. The run detector pump 22 applies regulated output pressure to the end area 124 through a direct path including the double check valve 136. The same regulatedpump pressure is applied to the opposite area 104 through an indirect path leading through the check valve 132, the line J connected to the valve 20, and the valve b which occupies a leftward tilted position from the vertical position as shown in FIG. 4 in solid lines, and thence through the line R and check valve 102 to the area 104.  
  The second phase of operation of the valve CD when it is shifted leftwardly from the position shown in solid lines in FIG. 4, occurs because the prior pressure against area 104 is diverted when the valve output is transferred to the line F, in attendance with the low pressure control signal on the pressure movable area 104 is subjected by the double check valve 102 to the low pressure side of the pump 22 and to the low pressure or drain side of the valve 20.  
  For its third phase of operation in which the valve spool element CD is&#39;recentered to its solid line position as shown in FIG. 4, the pressure area 124 likewise is relegated, by the double check valve 136, to a reduced pressure signal from the output side of the pump 22 and to a reduced signal from the side of the valve 20 supplying the line F. In the third phase, the slow running run detector pump 22 cannot maintain pressure above a predetermined reduced pressure because the restriction in the pressure regulating circuit 134 handles the output unassisted and without build-up of appreciable back pressure across the restriction.  
  The converse result from the foregoing can readily be visualized, namely, the three similar phases which occur when the vehicle is rolling forwardly under power and the control is reversed to a rear drive position. Circuit symmetry assures such result, the rolling vehicle in either event being immediately declutched and progressively braked, and subsequently being immediately unbraked and progressively clutched to drive in the opposite direction.  
  It is significant in my invention that the transmission maintains disengagement only a minimum period during reversal of movement, which is valuable under the circumstances of rapid to-and-fro movement such as a crawler vehicle is utilized for. The spring tension in the recentering springs 138 and 122 is such that at a point prior to completion of the braking down of the vehicle, the compressed one of those springs 138 or 122 will overcome the diminishing pressure from the pump 22 and complete the power shift. Thus the selected driving clutch aids in reversing the vehicle direction by a somewhat premature engagement, either with or without assist from the brake cylinder 26 which, at some point, is disengaged completely. This invention in the broad sense contemplates a slight overlap between the selected clutch and the brake cylinder 26 in their operation, all prior to the point at which the vehicle transitorily reaches actual standstill.  
  The brakes have two ways of releasing. In the way referred to in the immediately preceding paragraph, the transmission has been caused to undergo reverse engagement. That is, after the shift control handle 72 has been reversely shifted, the valve CD eventually centers with gradual slow-down of the pump 22, thereupon restoring brake release pressure in the line 108 so that the cylinder 26 in brake-release position will disengage braking.  
  In the other way as set out in the preceding discussions under the neutralization headings, I detailed that retention of the selector lever 72 in each of the three neutral positions, or movement of the lever into but not past the neutral positions results in automatically holding the cylinder 26 in the brake-release position. Yet conventional manual controls remain available to set the brakes under those circumstances, as by pedal actuation or handle (H) actuation, with or without power assist such as from a brake booster cylinder (not shown) or from the cylinders 50 or 26 as shown.  
  A regulator valve 164 (FIG. 4) maintains essentially constant intermediate or lube pressure L in the lube pressure chamber 116 of the valve 78. The valve 156 and one of the valves 154 of the two pairs of check valves concerned have a common inlet-outlet port 166 to which they are connected by an intervalve communication. The valve 132 and the other valve 154 of the same two respective pairs of valves have a common inlet-outlet port 168 to which they are connected by an inter-valve communication. The pair of valves 132 and 156 have a first outlet J common to them in the valving system.  
  The double check valve 102 is connected to the same inlet-outlet port 166 and has a second outlet 170 in the system communicating with the fluid inside the pressure chamber 106 in the valve 78. The double check valve 136 is connected to the other inlet-outlet port 168 and has a third outlet 172 communicating with the fluid inside another pressure chamber in the valve 78.  
  Variations within the spirit and scope of the invention described are equally comprehended by the foregoing description.  
 What is claimed is:  
 I. In combination:  
 a control valve assembly comprising a housing having high (126), transfer (110), and low (114, 116) pressure chambers;  
 a common bore intersecting the chambers;  
 individual valve bores in communication with the transfer and low pressure chambers and with plural service passages (84, 86, 88, in said housing;  
 first and second range selector valve elements F1F2, R2R1) each shiftably disposed in a different one of the individual valve bores, said valve elements being movable selectively from a position interconnecting the low pressure chamber and the service passages to discrete operating positions at once interconnecting the transfer chamber and selected service passage and interconnecting the low pressure chamber and the unselected service passages;  
 a dump valve element (CD) shiftably disposed in said common bore and having an undisplaced, satisfied position interconnecting the transfer and high pressure chambers and a displaced, dump position interconnecting the transfer and low pressure chambers;  
 means including pressure movable control means (104, 124) carried by the dump valve element for shifting same selectively to make available high pressure to, or to dump the pressure from, the transfer passage and the selected service passage;  
 utilization means for said control valve assembly comprising a vehicle supported for rolling movement on a horizontal surface;  
 a run detector dump (22) connected to the vehicle to sense its direction of rolling movement and having a directionalized pump output;  
 means including a desired direction selector (20) for controlling application of the pump output in two paths to said pressure movable means carried by the dump valve element each opposing and balancing the other so that said element takes an undisplaced, satisfied position when the direction in which the vehicle is rolling as sensed by the pump is the desired direction;  
 said application means including the desired direction selector comprising valving including a neutralizable operator operated valve effective to communicate pump output to an unopposed, valve element displacing, pressure movable means carried by the dump valve element when the direction in which the vehicle is rolling as sensed by the pump is undesired;  
 said dump valve element having means including a control device therefor (138 or 122) effective to restore the dump valve element to the undisplaced, satisfied position, when the pump output falls to or below a predetermined reduced value indicative that rolling of the vehicle in the undesired direction has reached a substantial standstill;  
 a brake operating cylinder for brakes carried by the vehicle; and  
 a communication between the brake operating cylinder and said transfer chamber to make available high pressure to, or to dump the pressure from, the cylinder for automatically holding the brakes set when the direction in which the vehicle is rolling as sensed by the pump is the undesired direction, or for automatically holding the brakes released both when the direction in which the vehicle is rolling as sensed by the pump is the desired direction and also when the direction in which the vehicle is rolling as sensed by the pump is the undesired direc-- tion in the event that the pump output is at or below said predetermined reduced value 2. The invention of claim 1, said communication including:  
 orifice means (158) communicating the high pressure to said cylinder with resistance so as to restrict the rate of brake application in setting the brakes; and  
 a check valve (100) communicating the dumped pressure away from said cylinder and unseating in a direction for immediate brake release.  
 3. In combination:  
 a control valve assembly comprising a housing having high (126), transfer (110), and low (114, 116) pressure chambers;  
 a common bore intersecting the chambers;  
 a dump valve element (CD) shiftably disposed in said common bore and having an undisplaced, satisfied position interconnecting the transfer and high pressure chambers and displaced, dump position interconnecting the transfer and low pressure chambers;  
 means including pressure movable control means pressure to, or to dump the pressure from, the transfer passage and the selected service passage;  
 utilization means for said control valve assembly comprising a vehicle supported for rolling movement on a horizontal surface;  
 a run detector pump (22) connected to the vehicle to sense its direction of rolling movement and having a directionalized pump output;  
 means including a desired direction selector (20) for controlling application of the pump output in two paths to said pressure movable means carried by the dump valve element each opposing and balancing the other so that said element takes an undisplaced, satisfied position when the direction in which the vehicle is rolling as sensed by the pump is the desired direction;  
 said application means including the desired direction selector comprising valving including a neutralizable operator operated valve (20) effective to communicate pump output to an unopposed, valve element displacing, pressure movable means carried by the dump valve element when the direction in which the vehicle is rolling as sensed by the pump is undesired;  
 said dump valve element having means including a control device therefor (138 or 122) effective to restore the dump valve element to the undisplaced, satisfied position, when the pump output falls to or below a predetermined reduced value indicative that rolling of the vehicle in the undesired direction has reached a substantial standstill;  
 a brake operating cylinder for brakes carried by the vehicle; and  
 a communication between the brake operating cylinder and said transfer chamber to make available high pressure to, or to dump the pressure from, the cylinder for automatically holding the brakes set when the direction in which the vehicle is rolling as sensed by the pump is the undesired direction, or for automatically holding the brakes released both when the direction in which the vehicle is rolling as sensed by the pump is the desired direction and also when the direction in which the vehicle is rolling as sensed by the pump is the undesired direction in the event that the pump output is at or below said predetermined reduced value.  
  4. The invention of claim 3, said communication including:  
 orifice means (158) communicating the high pressure to said cylinder with resistance so as to restrict&#39; a direction for immediate brake release.