first inlet port 18 the second larger diameter region 16 is accessible through a first check valve 22 and an inlet or refill port 24 the first check valve 22 is configured to allow fluid flow from an exhaust feed line into the second larger diameter region 16 but not out of it the second larger diameter region 16 is also accessible through a second check valve 26 and outlet or exhaust port 28 the second check valve 26 is configured to allow fluid flow out of the second larger diameter region 16 but not into it the first and second check valves 22 and 26 may be any suitable type of one way flow valves such as the ball type check valves illustrated a flapper valve or a poppet valve a stepped or dual diameter piston 30 is slidingly disposed within the stepped or dual diameter housing or cylinder 12 and a first compression spring 32 is disposed within the second larger diameter region 16 and biases the dual diameter piston 30 toward the first inlet port 18 a spool valve 40 includes a generally cylindrical housing 42 which slidingly receives a valve spool 44 the valve spool 44 includes a plurality of spools or lands 46 the cylindrical housing 42 defines an inlet port 48 a supply port 52 which communicates with a branching supply line 54 and an exhaust port 56 the inlet port 48 is connected to a source of pressurized hydraulic fluid or oil such.