Pump valve rebuild kits available. Or the pump may just need to be cleaned out. Debris in the rubber valves.
Do you have a strainer housing on the inlet side of the pump ?
Worth installing one if you don't have.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.