chamber 30 and a modulator valve 27 keeps a high setting pressure with discharging a little drain oil when the pedal stroke s increases to a value s1 near the maximum value and the clutch pressure decreases to the switching value p1 the passage 35 connects to the tank t through a space in which a spring 42 is arranged so that the pressure in the chamber 30 is reduced and the setting pressure in the modulator valve 27 is also reduced when the brake pedal 20 is returned the clutch pressure p gradually increases however since the modulator valve 27 keeps the low setting pressure with discharging much oil through the drain passage 34 increasing rate of the clutch pressure as well as the pressure in the chamber 45 is low therefore even after the pedal stroke s decreases below the switching value s1 the spool 39 does not return to a position in which the passage 35 is connected to the passage 48 so that the pressure in the chamber 30 is kept in reduced condition thus the valve 27 still keeps the low setting value with discharging much oil and the clutch pressure p slowly increases as shown in a curve y in fig7 when the pedal stroke s decreases to a switching value s 39 1 and the clutch pressure p increases to the switching value p1 the spool 39 and 40 reaches a position in which the passages 35 and 48 are connected together through the annular groove 37 so that the pressure.