A retainer nut assembly for a fluid end of a pump system includes a fastener with a generally cylindrical configuration, first and second ends, external threads configured to engage corresponding threads of a fluid end block, and a cavity formed at the first end. A suction cap is configured to sealingly fit to the fluid end block. A load piston is movably disposed in the cavity adjacent the suction cap. A bore is formed through the fastener in communication with the cavity and provided with hydraulic fluid. A pressure piston is movably disposed in the bore. A threaded passageway is formed through the fastener in communication with the bore and open to the second end. A locking bolt is disposed in the threaded passageway and, when inserted into the threaded passageway, contacts the pressure piston and generates fluid pressure on the load piston.