PURPOSE: To make a boost sensitive differential pressure regulating valve operatable normally at all times, by installing an accumulator in a passage which leads operation controlling hydraulic pressure in the boost sensitive differential pressure regulating valve controlling sealed-in pressure in a control valve, while absorbing a pressure variation in the operation controlling hydraulic pressure.

CONSTITUTION: Sealed-in pressure in a chamber 30 controlling a proportioning valve (P valve) 3 is controlled by a boost sensitive differential pressure regulating valve 5 which is, in turn, controlled by differential pressure in front and rear chambers 48 and 49 of an orifice 50 installed in its plunger 40. And, the chamber 48 is connected to a circuit in a rear brake 61 system via a branch passage 51. In this branch passage 51, there is provided with an accumulator 100, and in time of the P valve being closed by the accumulator 100, pressure variations produced in the rear brake 61 system are absorbed. Accordingly, the hydraulic pressure led into chamber 48 is no longer affected by these pressure variations so that the boost sensitive differential pressure regulating valve 5 normally operates at all times.