PURPOSE: To facilitate manual brake control by closing a swtich valve installed between a braking unit and a servo-brake liquid pressure generating chamber with a specific control piston.

CONSTITUTION: A servo-control system 13 is provided with a valve chamber 15 and a servo-brake liquid pressure generating chamber 16 mutually partitioned. A control piston 17 having a large-sized portion 17a and a small-sized portion 17b is fitted inside the chamber 15, and liquid chambers 18 and 19 are provided on both right and left sides of the piston and liquid inlet and outlet nozzles 20 and 21 are bored and connected to a master cylinder 42 and a brake cylinder 10 respectively. The left end face of a valve disk 23 which opens can closes a communicating nozzle 22 between the chambers 18 and 19 is energized by a spring 24, to a plunger 25 facing the chamber 18, and a communicating channel 34 between the chambers 16 and 19 is opened and closed by a check valve 35 which can contact the control piston 17. With the liquid pressure in the chamber 18 in proportion to the grip of a lever 44, the plunger 25 first moves to the right and then the piston 17 repeatedly moves both left and right, thereby increasing the internal pressure in a brake cylinder liquid chabmer 11.