a fluid pressure energy translating device of the sliding vane type comprising a cam ring including an internal contour , a rotor having a plurality vanes rotatable therewith and slidable relative thereto in slots in the rotor with one end of each vane engaging the internal contour . the rotor and internal contour cooperate to define one or more pumping chambers between the periphery of the rotor and the cam contour through which the vanes pass carrying fluid from an inlet port to an outlet port . two pressure chambers are formed for each vane and each vane has two surfaces one in each chamber , both being effective under pressure in the respective chambers to urge the vanes into engagement with the cam . pressure sensing passages extend from the periphery of the rotor to one of the chambers to provide pressure to the chamber . the end of each vane is tapered with the radially outermost portion of the end extending in a trailing manner and each pressure sensing passage leads the respective vane with respect to the direction of rotation thereby sensing pressure ahead of each respective vane . the leading passages also provide paths for exhausting the undervane displacement to ensure hydrostatic bias on the vane and cause the vanes to contact the cam contour during the pressure transition and displacement zones .