PURPOSE: To shorten a time required until the communication of an oil supply port with a brake port by carrying out the displacement of a spool quickly in emergent braking.

CONSTITUTION: The inner part of a rear cushion chamber 29 is communicated with a brake port 16 by a feedback oil passage 33 provided a throttle on the way. An exhaust oil passage 43 is provided in a rear cushion plunger 30. In emergent braking, the operation oil in the rear cushion chamber 29 is exhausted from not only the feedback oil passage 33 but the exhaust oil passage 43. Consequently, a time required in the push in of the rear cushion plunger 30 is shortened and the displacement of a spool 19 can be carried out quickly.