positioned in bypass passage 72 and is formed with a ball type valve which is engageable with a valve seat 78 in the absence of any biasing force by a spring when an oil pressure in the oil passage 73 in communication with inlet port f of modulator valve 60 is higher than the oil pressure in the oil passage 74 in communication with outlet port c of change over valve 50 the one way check valve 71 is disengaged with the valve seat 78 so that the oil pressure in the passage 73 in communication with an accumulating chamber 64b in the modulator valve 60 is drained to the upstream side oil passage 74 of third oil passage 34 the oil passage 32 communicates with the other inlet ports a and b of modulator valve 60 via the fourth oil passage 32 39 each of the drain ports c d and e of modulator valve 60 communicate with an oil sump 2 via a drain passage 35 both drain ports a and e of selector valve 40 communicate with the oil sump 2 via a drain passage 35 drain ports c and d of regulator valve 10 communicate with the oil sump 2 via the oil drain passage 35 the oil pressure in the passage 31 is from regulator valve 10 and is regulated to an opening degree between an oil chamber 16 formed within regulator valve 10 and a valve land 12 formed on the valve spool 11 of regulator valve 10 wherein the valve spool 11 is operated by a balancing force defined by a biasing force of a spring 15 and an oil pressure operating to valve lands 12 13 and 14 formed on valve spool 11 as an input oil pressure of regulator valve.