referring to fig1 the booster includes a housing 1 having a fluid port 2 connected with a pressurized fluid accumulator ( not shown ), port 2 being permanently subjected to pressure . housing 1 includes therein a booster piston 3 pressure - sealed to and slidable in housing 1 by means of the seals 4 and 5 . between seals 4 and 5 booster piston 3 has a radial groove 6 which creates an inlet chamber 7 between booster piston 3 and housing 1 , which is supplied with pressurized fluid via fluid port 2 . booster piston 3 is stepped with its smaller diameter section 8 extending out of housing 1 . a seal 9 seals section 8 to housing 1 to seal the inner chamber of housing 1 from the atmosphere . provided in booster piston 3 is a centrally arranged , stepped bore 10 , open in the direction of the tappet 11 . central piston 13 slides in the smaller diameter section 12 of stepped bore 10 , with section 12 of bore 10 being connected with a pressureless chamber 15 via a radial passage 14 . the pressureless chamber 15 is in direct communication with a reservoir ( not shown ), from which the pressure accumulator is fed . via bores 16 and 17 the inner chamber 18 of the larger diameter section of stepped bore 10 is connected with pressureless chamber 15 . via the openings 19 the pressure chamber 20 is also kept pressureless . pressure chamber 20 is bounded by the step 21 between the large diameter section of booster piston 3 and section 8 , by booster piston 3 itself and by housing 1 . the end part 22 of central piston 13 , exposed in inner chamber 18 , embraces the rim 23 of a pin 24 , which centrally penetrates the annular piston 25 . pin 24 is displaceable by tappet 11 relative to central piston 13 in the actuating direction 26 against a spring 27 , which is engaged between central piston 13 and pin 24 . the chamber 28 , which is a result of collar 30 embracing pin rim 23 , is directly connected with inner chamber 18 via the opening 29 . annular piston 25 on its part embraces collar 30 provided at end part 22 of central piston 13 in such a manner that central piston 13 and annular piston 25 are movable relative to each other within limits in the sense of a positive engagement clutch . annular piston 25 , which bounds inner chamber 18 towards tappet 11 and the atmosphere , slides in inner chamber 18 , sealed from the pressurized fluid by means of a seal 31 placed in booster piston 3 . annular piston 25 is constructed slightly stepped on its circumference , so that a chamber 32 is formed in booster piston 3 between seal 31 and step 35 of annular piston 25 . chamber 32 is in communication with inner chamber 18 of booster piston 3 via a longitudinal groove 33 in the outer circumference of annular piston 25 . chamber 32 serves for accommodation of a spring bearing to enable annular piston 25 to resiliently bear against booster piston 3 . this spring bearing includes a spring 34 abutting step 35 of annular piston 25 . spring 34 keeps a disc 36 in abutment with a projection 43 of booster piston 3 . when annular piston 25 is moving opposite to actuating direction 26 , spring 34 is compressed . spring 34 is relieved when annular piston 25 moves in actuating direction 26 , after disc 36 abuts projection 37 of annular piston 25 . further , annular piston 25 has a seal 38 , which seals annular piston 25 to pin 24 . on the end of annular piston 25 remote from inner chamber 18 , the spherical head 39 of tappet 11 is fixed in annular piston 25 . the fastening of head 39 in annular piston 25 is such that the end of head 39 adjacent pin 24 is at a distance x from where it abuts annular piston 25 . in all operating positions pin 24 is mechanically abutting head 39 of tappet 11 . central piston 13 has in inner chamber 18 about its circumference a collar 40 , which abuts a spring 41 bearing against the step 42 in booster piston 3 . springs 27 and 41 may be constructed and dimensioned such that central piston 13 will abut annular piston 25 in the inactive position , or the adjacent ends of central piston 13 and annular piston 25 will be spaced from each other . in the inactive position of the device as is shown , the inactive position of booster piston 3 is ensured by return spring 44 , which urges the step 45 of booster piston 3 into abutment with housing 1 . ensured by this position is also the position of central piston 13 , since return spring 41 of central piston 13 bears against booster piston 3 . since spring 27 has a greater rigidity compared with spring 41 , pin 24 will be in the position shown and , in doing so , will keep head 39 of tappet 11 in the right - hand position illustrated . the position of tappet 11 , which is mechanically connected with annular piston 25 , fixes annular piston 25 likewise in its inactive position . in the inactive position shown , radial bores 47 , which penetrate booster piston 3 and connect inner chamber 12 with inlet chamber 7 , are closed by central piston 13 . the radial bores 48 , arranged in central piston 13 and in communication with bore 16 are closed by booster piston 3 in the same way . to protect the parts arranged in inner chamber 18 of stepped bore 10 , a protecting cap 50 is provided , which is situated in a groove of housing 1 and sealingly embraces tappet 11 . the mode of operation of the device shall be explained with reference to the graph of fig2 . depression of brake pedal 46 causes tappet 11 in annular piston 25 to move relative to annular piston 25 and to displace pin 24 in actuating direction 26 . since spring 27 has a greater rigidity than spring 41 , central piston 13 is carried along in actuating direction 26 due to the displacement of pin 24 . due to this , central piston 13 will close opening 14 and separate inner chamber 18 from pressureless chamber 15 . at the same time bore 48 will come to lie under bore 47 , so that pressurized fluid will flow from inlet chamber 7 through bores 47 and 48 and bores 16 and 17 into inner chamber 18 . via opening 19 the rising pressure will be transmitted into pressure chamber 20 . the now prevailing pressure acts on booster piston 3 in the actuating direction 26 and will subject the piston of a master brake cylinder of a brake unit to a force , for example , via the force transmitting member 49 . at the same time the hydraulic pressure will act on annular piston 25 and displace annular piston 25 slightly against the force of spring 34 opposite to actuating direction 26 , until the pressure forces , which are acting on the effective surface in inner chamber 18 , are in balance with the force of spring 34 . being pressure - balanced , central piston 13 itself will not transmit any forces to its surroundings . only pin 24 will transmit the pressure forces acting on it to head 39 of tappet 11 and , thus , on to brake pedal 46 . the driver will consequently achieve a comparatively high brake pressure when using only low pedal force . this is particularly advantageous in heavy vehicles equipped with an automatic gearbox , where a relatively high pressure force has to be applied even with the vehicle at a standstill . the brake pressure produced can be seen in the graph of fig2 by the controlled delivery of the pedal force . the input pedal force , identified as k1 , at which there does not yet exist any brake pressure , is basically determined by spring 41 . according to the type of vehicle , the necessary input pedal force could be predetermined by installing a spring 41 having the desired force . the slope from a to b in the graph of fig2 is in the main determined by the hydraulic force , which counteracts the driver &# 39 ; s foot force when the hydraulic booster is actuated . as shown , the slope from point a to point b is relatively steep , since the effect of the hydraulic forces on the brake pedal is relatively small due to the diameter of pin 24 . in this case , too , a special slope from point a to point b , desirable for a specific type of vehicle , could be attained by structural modifications , for example , by choosing various diameters of pin 24 . at point b the hydraulic booster is balanced out to such an extent that head 39 of tappet 11 abuts the interior of annular piston 25 , thus , using up the predetermined free motion x . for the force to continue to rise at outlet member 49 of the hydraulic booster , however , the entire piston 25 has to now be displaced against the hydraulic force in inner chamber 18 . this results in considerably higher pedal forces , as seen in the graph of fig2 . in addition , this arrangement ensures that upon failure of the hydraulic booster , a mechanical actuation of the brake unit is possible . after the free motion x is overcome , annular piston 25 together with tappet 11 will be immediately urged into abutment with central piston 13 , which on its part may be urged into mechanical abutment with booster piston 3 . thus , a mechanical interconnection of force transmitting member 49 and brake pedal 46 is guaranteed . as soon as no more hydraulic force is applied on force transmitting member 49 , the pistons will return to their inactive position by means of the springs and , in doing so , will connect inner chamber 18 as well as pressure chamber 20 with pressureless chamber 15 again . while i have described above the principles of my invention in connection with specific apparatus , it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims .