I've replaced valves in 3 water pumps, if your able to do the work yourself ; and this is not complicated; a big $ savings.
Just a thought, do you have a pre pump screen , the housing on the screen can leak air and cause a loss of prime too. It's a screw on housing on the inlet side , clear plastic about 2" dia. worth a look.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.