PURPOSE: To prolong the lifetime of a brake lining through reduction of a load on the main braking system by enabling adding of the counter force generated by the operation of a brake pedal to a hydraulic pump connected to a motor.
CONSTITUTION: When a foot brake pedal 37 is pushed, a shift valve 70 is shifted to a position C2 and the hydraulic channel of direction shift valves 50 and 60 and a pump 32 is intercepted, while the oil from the pump 32 is returned to a relief valve 80 through the intermediary of pipes 36 and 45. However, since the valve 80 is left closed when the oil pressure is below a set pressure, the resistance due to the oil pressure in a hydraulic system increases and thereby the braking force is applied to the pump 32, i.e. the driving shaft 33 of an engine 3. Accordingly, when the above- mentioned pedal 37 is pushed down, the force of the engine brake is applied in addition to the origianl braking action by the supply of the brake oil pressure to wheel cylinders 95 and thereby the load on the main braking system can be reduced.