You could have a few issues.injector orings,bad ipr or ipr orings,sticking check valve in the hpop,back cover or line fitting leaks on the hpop,hpop reservoir gasket leak.
Pull the fuel filter and see what the fuel in the housing and the filter look like.black,oily,gritty is a good indication that you need injector orings
With the truck being a 95 and not knowing anything about the truck.it is possible that somebody swapped in 96-03 hpop wich will cause the hpop tank to drain over a couple days of sitting.95 has the check valve in the hpop,96-03 have it in the timing cover.