Ap2d21 Pump Repair Parts Valve Plate Excavator Pump Replaecment Parts Valve plate porting and commutation: As the cylinder block rotates, each piston chamber is automatically alternately connected to the high-pressure and low-pressure ports to complete oil suction and discharge, replacing a separate directional valve. Sealing and isolation: Forms an oil film friction pair with the end face of the cylinder block, isolates high-pressure and low-pressure chambers, and prevents