get a 3/8" round rod & grind/deburr/clean an end to a 5/16 hex, dont have 2 b perfect just work on it w your bench grinder till it fits fairly good & as said prime CCW. If no go might take off the filter & try to add a bit of oil in the pump & reprime w filter off (drape a rag over the pump) but w it loosing psi out of the clear blue sky it sure sounds like the prob is upstream almost like an inside pipe plug came out but I cant see that happening. Pull the pipe thread plug on the pressure regulator on the pump & see if plunger/spring is still free. Holler what it ends up being EDIT if you grind a rod look at an allen wrench the same size to get where you want it