referring to fig1 , there is shown a hydraulic control apparatus for a vehicle having an engine and an automatic transmission coupled to the engine , according to an embodiment of the present invention . the automatic transmission is equipped with belt - drive continuously variable transmission ( hereinafter referred to as cvt ) 3 . as illustrated in fig1 , cvt 3 is coupled to the engine via lockup clutch 2 for direct connection between cvt 3 and the engine . torque converter 1 is connected to output shaft 12 of the engine . forward - reverse switching mechanism 20 is disposed on an output side of torque converter 1 . forward - reverse switching mechanism 20 includes a planetary gear train , reverse brake 24 and forward clutch 25 . the planetary gear train includes ring gear 21 coupled to output shaft 12 of the engine , pinion carrier 22 , and sun gear 23 coupled to input shaft 13 of cvt 3 . reverse brake 24 is operative to fix pinion carrier 22 to a transmission case . forward clutch 25 is operative to couple input shaft 13 of cvt 3 and pinion carrier 22 with each other and acts as a start clutch . oil pump 8 acting as an oil pressure source is mechanically coupled to the engine and directly driven by the engine . cvt 3 includes primary pulley 30 a , secondary pulley 30 b and belt 34 connecting primary and secondary pulleys 30 a and 30 b to thereby transmit the rotation force of primary pulley 30 a to secondary pulley 30 b . primary pulley 30 a is disposed on a rear side portion of input shaft 13 . primary pulley 30 a includes fixed disk 31 rotatable together with input shaft 13 , and moveable disk 32 opposed to fixed disk 31 in an axial direction of input shaft 13 . fixed and moveable disks 31 and 32 have generally conical shapes and cooperate with each other to form a v - groove in which belt 34 is engaged . moveable disk 32 is axially moved on input shaft 13 by an oil pressure supplied to primary pulley cylinder chamber 33 . secondary pulley 30 b is disposed on driven shaft 38 . secondary pulley 30 b includes fixed conical disk 35 rotatable together with driven shaft 38 , and moveable disk 36 opposed to fixed disk 35 in an axial direction of driven shaft 38 . fixed and moveable disks 35 and 36 have generally conical shapes and cooperate with each other to form a v - groove in which belt 34 is engaged . moveable disk 36 is axially moved on driven shaft 38 by an oil pressure supplied to secondary pulley cylinder chamber 37 . a driving gear , not shown , is fixed onto driven shaft 38 . the driving gear is operative to drive a driving shaft connected to a wheel via a pinion on an idler shaft , a final gear and a differential gear . the rotation force outputted from output shaft 12 of the engine is transmitted to input shaft 13 of cvt 3 via torque converter 1 and forward - reverse switching mechanism 20 . the rotation force of input shaft 13 is successively transmitted to primary pulley 30 a , belt 34 , secondary pulley 30 b , driven shaft 38 , the driving gear , an idler gear , the idler shaft , the pinion , the final gear and the differential gear . upon thus transmitting the rotation force , moveable disk 32 of primary pulley 30 a and moveable disk 36 of secondary pulley 30 b are axially moved on input and driven shafts 13 and 38 , respectively , to change a width of the v - groove which extends in the axial direction of input and driven shafts 13 and 38 . a radius of curvature of a circular arc formed by v - belt 34 contacted with pulleys 30 a and 30 b is continuously varied by changing the v - groove width . a pulley speed ratio between the rotational speed of primary pulley 30 a and the rotational speed of secondary pulley 30 b , namely , a transmission ratio of cvt 3 , can be thus changed . the change of the v - groove width is conducted by controlling the oil pressure supplied to primary pulley cylinder chamber 33 or secondary pulley cylinder chamber 37 . the hydraulic control is performed by cvt controller or control unit 9 . a plurality of sensors are electronically connected to cvt controller 9 to detect engine operating conditions . the sensors includes primary pulley speed sensor 4 , secondary pulley speed sensor 5 , throttle position sensor 10 and pulley clamping pressure sensor 14 . primary pulley speed sensor 4 detects the rotational speed of primary pulley 30 a and generates signal np indicative of the detected primary pulley speed . secondary pulley speed sensor 5 detects the rotational speed of secondary pulley 30 b and generates signal ns indicative of the detected secondary pulley speed . throttle position sensor 10 detects an opening degree of a throttle valve and generates signal tvo indicative of the detected throttle opening degree . pulley clamping pressure sensor 14 detects a pulley clamping pressure supplied to each of primary and secondary pulley cylinder chambers 33 and 37 for clamping belt 34 , and generates signal cp indicative of the detected pulley clamping pressure . cvt controller 9 receives the signals generated from these sensors , processes the signals , and develops and transmits control signal cs to hydraulic control valve unit 6 . cvt controller 9 may be a microcomputer including central processing unit ( cpu ), input and output ports ( i / o ), read - only memory ( rom ), random access memory ( ram ) and a common data bus . hydraulic control valve unit 6 receives a plurality of signals indicative of an accelerator opening degree , the transmission ratio of cvt 3 , the rotational number of input shaft 13 , a primary pulley pressure , and the like . hydraulic control valve unit 6 controls the transmission ratio of cvt 3 by supplying pulley clamping pressure cp to primary and secondary pulley cylinder chambers 33 and 37 based on the input signals . fig2 shows a hydraulic circuit used in the embodiment of the hydraulic control apparatus . as illustrated in fig2 , pressure regulator valve 40 is connected to oil pump 8 via oil passage 41 . pressure regulator valve 40 regulates a discharge pressure produced from oil pump 8 and outputs the regulated pressure as line pressure ( pulley clamping pressure ). oil passage 43 is communicated with oil passage 41 and supplies pulley clamping pressure cp to secondary pulley cylinder chamber 37 . oil passage 42 as a line pressure supply passage is communicated with oil passage 41 and supplies primary pulley cylinder chamber 33 with pulley clamping pressure cp for clamping belt 34 and a transmission control pressure for controlling the transmission ratio of cvt 3 . transmission control valve 50 is disposed on oil passage 42 and controls an oil pressure within primary pulley cylinder chamber 33 . namely , transmission control valve 50 controls supply and drain of an oil pressure relative to primary pulley cylinder chamber 33 . specifically , transmission control valve 50 has port 51 connected to oil passage 42 , port 52 connected to oil passage 52 a which is communicated with primary pulley cylinder chamber 33 , and port 53 connected to oil passage 72 which is communicated with oil cooler 80 . transmission control valve 50 also has spool 50 a moveable to switch fluid communication between ports 51 , 52 and 53 . spool 50 a has a block position where the fluid communication between ports 51 , 52 and 53 is prevented , a high speed side transmission position where ports 51 and 52 are fluidly communicated with each other to change the transmission ratio to a smaller value , namely , to the high speed side , and a low speed side transmission position where ports 52 and 53 are fluidly communicated with each other to change the transmission ratio to a larger value , namely , to the low speed side . spool 50 a is connected with stepping motor 90 and moveable disk 32 of primary pulley 30 a via link 91 . spool 50 a , stepping motor 90 and moveable disk 32 form a mechanical feedback mechanism for controlling the movement of spool 50 a . stepping motor 90 which acts as a transmission actuator for actuating spool 50 a , is driven based on a transmission control signal transmitted from cvt controller 9 . the transmission control signal includes a high speed transmission control signal for changing the transmission ratio to the high speed side and a low speed transmission control signal for changing the transmission ratio to the low speed side . stepping motor 90 actuates spool 50 a to shift between the high speed side transmission position and the low speed side transmission position in response to the high speed transmission control signal and the low speed transmission control signal . depending on these positions of spool 50 a , the oil pressure is supplied to primary pulley cylinder chamber 33 and discharged therefrom . this causes moveable disk 32 to axially move to change the width of the v - groove of primary pulley 30 a . in response to the change in the v - groove width , spool 50 a connected with moveable disk 32 via link 91 is returned to the block position . in the block position , the supply of the oil pressure and the discharge thereof are stopped and thus the transmission operation is completed . the operation of the mechanical feedback mechanism will be in detail explained later . an oil pressure drained from pressure regulator valve 40 is supplied to clutch regulator valve 60 downstream of pressure regulator valve 40 via oil passage 46 . oil passage 46 is communicated with oil passage 44 which is communicated with oil passage 42 and has orifice 45 . clutch regulator valve 60 regulates the oil pressure in oil passage 46 and the oil pressure in oil passage 61 to produce forward clutch applying pressure cap . forward clutch applying pressure cap is supplied to forward clutch 25 of forward - reverse switching mechanism 20 via oil passage 61 , a select switching valve , not shown , and a select control valve , not shown . with this arrangement , forward clutch applying pressure cap is regulated smaller than pulley clamping pressure cp . an oil pressure drained from clutch regulator valve 60 is supplied to torque converter regulator valve 70 downstream of clutch regulator valve 60 via oil passage 62 . torque converter regulator valve 70 regulates the oil pressure in oil passage 62 to produce a torque converter pressure and a lockup clutch applying pressure and a lockup clutch releasing pressure . the lockup clutch applying pressure and the lockup clutch releasing pressure are supplied to a lockup control valve via oil passage 63 communicated with oil passage 62 . an oil drained from torque converter regulator valve 70 is supplied to oil cooler 80 via oil passage 71 . the oil passing through oil cooler 80 is cooled and supplied to lubrication parts of cvt 3 to be lubricated . oil passage 71 is connected with oil passage 72 communicated with port 53 of transmission control valve 50 . fig3 illustrates the mechanical feedback mechanism for controlling spool 50 a of transmission control valve 50 . referring to fig3 , the operation of the feedback mechanism is explained . first , an operation of changing the transmission ratio to the low speed side , namely , an operation of increasing the transmission ratio , is described . in this case , the width of the v - groove of primary pulley 30 a must be increased by draining the oil pressure within primary pulley cylinder chamber 33 . first , stepping motor 90 is moved upward as viewed in fig3 . following the movement of stepping motor 90 , spool 50 a is also upward moved to the low speed side transmission position where port 52 is fluidly communicated with port 53 to thereby allow drain of the oil pressure within primary pulley cylinder chamber 33 from port 53 . the oil pressure drained is then supplied to oil cooler 80 via oil passage 72 as shown in fig2 . owing to the drain of the oil pressure within primary pulley cylinder chamber 33 , moveable disk 32 is moved downward as viewed in fig3 , to thereby increase the width of the v - groove . the transmission ratio is shifted to the low speed side . the downward movement of moveable disk 32 causes spool 50 a to move downward to the block position where the fluid communication between port 52 and port 53 is prevented . as a result , the drain of the oil pressure within primary pulley cylinder chamber 33 is stopped and the change of the transmission ratio to the low speed side is completed . next , an operation of changing the transmission ratio to the high speed side , namely , an operation of decreasing the transmission ratio , is explained . in this case , the width of the v - groove of primary pulley 30 a must be decreased by supplying an oil pressure to primary pulley cylinder chamber 33 . when stepping motor 90 is moved downward as viewed in fig3 , spool 50 a is also moved downward to the high speed side transmission position where port 51 is fluidly communicated with port 52 to thereby allow supply of the oil pressure from pressure regulator valve 40 to primary pulley cylinder chamber 33 . this causes moveable disk 32 to move upward as viewed in fig3 , to thereby decrease the width of the v - groove . thus , the transmission ratio is shifted to the high speed side . the upward movement of moveable disk 32 causes spool 50 a to move upward to the block position where the fluid communication between port 51 and port 52 is prevented . as a result , the supply of the oil pressure to primary pulley cylinder chamber 33 is stopped and the change of the transmission ratio to the high speed side is completed . when the vehicle is stopped while the transmission ratio is kept on the high speed side , spool 50 a is placed in the block position where port 52 is prevented from fluid communication with port 51 and port 53 . in this condition , when the engine is restarted and stepping motor 90 is actuated to drive spool 50 a for changing the transmission ratio to the low speed side , port 52 and port 53 are communicated with each other . therefore , the oil pressure to be supplied from torque converter regulator valve 70 toward oil cooler 80 via oil passage 71 is supplied to primary pulley cylinder chamber 33 via oil passages 71 and 72 , ports 53 and 52 , and oil passage 52 a . this can establish the oil pressure , namely , pulley clamping pressure cp , within primary pulley cylinder chamber 33 , which is required to clamp belt 34 , to thereby prevent occurrence of slippage of belt 34 . this serves for improving durability of belt 34 . further , the oil pressure regulated by clutch regulator valve 60 is smaller than the oil pressure regulated by pressure regulator valve 40 . the oil pressure regulated by torque converter regulator valve 70 is smaller than the oil pressure regulated by clutch regulator valve 60 . port 53 is connected with oil passage 72 connected with the downstream side of pressure regulator valve 40 . specifically , oil passage 72 is connected with oil passage 71 for supplying the oil pressure from torque converter regulator valve 70 to oil cooler 80 . with the arrangement , in an ordinary case where the transmission ratio is on the low speed side , the oil can be discharged from port 53 without adversely influencing other hydraulic control circuits . on the other hand , in a case where the oil pressure within primary cylinder chamber 33 becomes lower than a minimum oil pressure required for clamping belt 34 , for instance , upon the vehicle stopping while the transmission ratio being kept on the high speed side , upon the occurrence of failure of transmission actuator 90 , and the like , the oil discharged from torque converter regulator valve 70 toward oil cooler 80 and the lubrication parts can be bypassed and supplied to primary cylinder chamber 33 via oil passages 71 and 72 and ports 53 and 52 . this can establish the oil pressure within primary cylinder chamber 33 which is sufficient to clamp belt 34 and prevent slippage of belt 34 . further , the connection between port 53 and the oil passage downstream of pressure regulator valve 40 is not limited to the above embodiment . port 53 may be connected with other oil passages downstream of pressure regulator valve 40 in which the oil pressure smaller than the line pressure produced by pressure regulator valve 40 but larger than the minimum oil pressure required for clamping belt 34 is established . for example , as illustrated by broken lines in fig2 , port 53 may be connected with oil passage 63 via oil passage 172 or may be connected with oil passage 61 via oil passage 272 . in such a case , the same effect as described above can be performed . this application is based on a prior japanese patent application no . 2002 - 285499 filed on sep . 30 , 2002 . the entire contents of the japanese patent application no . 2002 - 285499 is hereby incorporated by reference . although the invention has been described above by reference to a certain embodiment of the invention and the modifications , the invention is not limited to the embodiment and modifications described above . modifications and variations of the embodiment and modifications described above will occur to those skilled in the art in light of the above teachings . the scope of the invention is defined with reference to the following claims .