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 number of by-pass passages enable the rotor bores to communicate with the high pressure port before being in full alignment therewith. 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 of these pressures causes variation of the position, relative to top-dead-center, at which successive rotor bores communicate with the high pressure port via the by-pass passages. The arrangement is such that pressures in the rotor bores and the high pressure port, immediately before they communicate, are maintained substantially equal over a range of pressures.