PURPOSE: To sense the start of operation of a retarder and improve safety by installing a pair of pressure adjusting valves in a reservoir tank and generating a high pressure for the time corresponding to the magnitude of the stationary brake torque in two stages which are generated by a fluid charge type reservoir connected with the tank.

CONSTITUTION: A throttle valve 17 and a check valve 20 are installed through a selector valve 11 on a reservoir tank 10 connected with a fluid charge type retarder 1, and the throttle valve 17 is connected with a pressure air source 12. The stationary brake torque in two stages can be generated in the retarder 1 by the actions of the throttle valve 17 and the check valve 20. Each pressure adjusting speed of the check valve 20 and the throttle valve 17 differs, and the check valve 20 can complete pressure adjustment speedily for the pressure over a stationary pressure, by a spring 20a, and the throttle valve 17 carries out pressure adjustment gradually through throttle, and the pressure higher than each stationary pressure in two stages is generated for the time corresponding to the magnitude of the stationary pressure. Accordingly, a driver can easily senses the start of operation of the retarder 1, and correct brake application is permitted.