Abstract: A rotary hydraulic pump or motor has a piston-carrying rotor having a face engaging a port plate which includes a high pressure port. A valve arrangement is responsive to the pressures in the high pressure port and in successive ones of the rotor bores to provide that variation in these pressures causes variation of the position, relative to top-dead-center, at which successive rotor bores communicate with the high pressure port through by-pass passages, and thereby substantially equalize the pressures in the bores and the high pressure port. The port plate and rotor also include passages by which the pressures in the rotor bores can be increased rapidly after their respective top-dead-center positions.