91 lumina loses fuel pressure

Hello all,
New to the forums and have been searching for a cure to this problem. Have a 3.1 with 240K that I picked of for my daughter as a work car. When I got car previous owner said that a new timing chain / gear set were installed and that they could not get the car running again. Did the basic checks and found some bent valves. Pulled heads had work done and reinstalled. Got the car running. The problem I am having now is that the fuel pressure at the rail will cycle to 40 pounds, car starts and runs great untill you shut it off. the fuel pressure drops to zero in about 4-5 seconds. Thought fuel pressure regulator went bad. Replace it and the injector o-rings since I was in there. Also changed the fuel filter. Still have the same problem. Have to cycle the fuel pump 5-6 times to get fuel pressure to start and run. Any ideas were to look next. Previous owner said the fuel pump was replaced about 6 month ago. Is there possibly a check valve in the sending unit/fuel pump that is letting the car bleed down so fast.

Welcome to AF, make sure no leaks in the system first, then focus on the pump.