Hydraulic pumps and motors

Claim 1

A HYDRAULIC PUMP COMPRISING A ROTATABLE HOUSING AND A PORT PLATE, SAID HOUSING AND PORT PLATE HAVING ABUTTING SURFACES, SAID PORT PLATE HAVING TWO PORTS LOCATED ON RELATIVELY OPPOSITE SIDES OF A PLANE CONTAINING THE AXIS OF RELATIVE ROTATION, ONE PORT BEING ARRANGED TO HOLD LIQUID UNDER A RELATIVELY HIGH PRESSURE DURING OPERATION AND THE OTHER BEING ARRANGED TO HOLD LIQUID UNDER A RELATIVELY LOW PRESSURE, SAID HOUSING HAVING A PLURALITY OF CHAMBERS CONTAINING RECIPROCATING PLUNGERS, EACH CHAMBER COMMUNICATING WITH THE SURFACE OF THE HOUSING BY WAY OF A PASSAGE, THE MOUTH OF EACH PASSAGE OF SAID SURFACE BEING FORMED WITH A PROJECTING RECESS LOCATED ON A RADIAL SIDE OF SAID MOUTH WITH RESPECT TO THE AXIS OF RELATIVE ROTATION, A DUCT IN SAID PORT PLATE WHICH OPENS ONTO THE SAID SURFACE BEING SO ARRANGED THAT DURING A PERIOD IN WHICH A CHAMBER IS IN COMMUNICATION WITH NEITHER OF SAID PORTS DURING OPERATION, THE RECESS ASSOCIATED WITH THE MOUTH OF THE CORRESPONDING PASSAGE COMMUNICATES WITH SAID DUCT OPENING ONTO THE SURFACE OF THE PORT PLATE, SAID DUCT BEING IN COMMUNICATION BY WAY OF A FURTHER DUCT WITH A FURTHER RECESS PROVIDED IN ONE OF SAID SURFACES, WHEREBY A CHAMBER IS IN COMMUNICATION WITH THE FIRST MENTIONED DUCT VIA THE CORRESPONDING PASSAGE AND THE RECESS ASSOCIATED WITH THE MOUTH OF SAID CORRESPONDING PASSAGE AND LIQUID PASSES FROM THE CHAMBER TO THE FURTHER RECESS.