Servobrake comprising a reduced-reaction master cylinder

A servobrake (10) for a motor vehicle, of the type comprising a pneumatic brake booster (12) a control rod (14) of which is able, in response to an input force, to indirectly control a pneumatic piston (16) and/or directly control a feeler (18) coaxial with the pneumatic piston (16), which pneumatic piston (16) and feeler (18) are intended to act upon a main piston (20) of a hydraulic brake master cylinder (22), of the type in which the main piston (20) is mounted to slide inside a roughly axial bore (32) of a body (30) of the master cylinder (22) between a rear position of rest and a forward position of applying a braking force, of the type in which the main piston (20) is elastically returned toward its rear position of rest, of the type in which at least one sealing means (38) is interposed between the main piston (20) and the bore (32) to delimit, within the bore (32), a rear supply chamber (36) supplying hydraulic fluid and a front pressure chamber (24), of the type in which the main piston (20) comprises at least one main valve shutter (44) which can move between an open position, corresponding to the rear position of rest of the main piston (20), whereby the main valve shutter (44) places the rear supply chamber (36) and the front pressure chamber (24) in communication, and a pressurizing position, corresponding to the forward position of application of the main piston (20), whereby the main valve shutter (44) interrupts the communication between the rear supply chamber (36) and the front pressure chamber (24) to allow a hydraulic braking pressure to build up in the front pressure chamber (24), of the type in which the main piston (20) comprises:

• a first rear end (46), secured to the main piston (20), which is intended to be actuated by the pneumatic piston (16) of the booster (12),
• a second rear end (48) which is intended to be actuated by the feeler (18) coaxial with the piston (16) and which is secured to a piston (50) known as a reaction piston of a ram (52) which is housed in the main piston (20), which ram (52) comprises a chamber (54) known as a reaction chamber able to be placed in communication with the front pressure chamber (24) of the master cylinder to transmit, to the second rear end (48), the resultant reaction of the pressure forces in the front pressure chamber (24),
• isolation means, which are interposed between the reaction chamber (54) of the ram (52) and the front pressure chamber (24), and which are able, in response to the advance of the second rear end (48) with respect to the first rear end (46) when the input force exerted on the control rod (14) of the booster (12) exceeds a given rate threshold, to isolate the reaction chamber (54) of the ram (52) from the front pressure chamber (24) such that the resultant reaction of the pressure forces in the front pressure chamber (24) is not transmitted to the second rear end (48), characterized in that the isolation means comprise:
• a hydraulic isolation valve shutter (58) which is interposed between the reaction chamber (54) of the ram (52) and the front pressure chamber (24) and which is independent of the reaction piston (50) of the ram (52);
• and
• flow restricting means, interposed between the hydraulic isolation valve shutter (58) and the front pressure chamber (24), in order, when the pressure in the reaction chamber (54) of the ram (52) exceeds a given pressure threshold associated with the exceeding of the given rate threshold, to close the hydraulic isolation valve shutter (58) so as to interrupt the communication between the reaction chamber (54) of the ram and the front pressure chamber (24).