Fuel Pump Question

[4-cam]

Do the fuel pumps have a check valve in them? While trouble shooting a code 55 after changing injectors, I was monitoring the fuel pressure and as soon as I shut it down the fuel pressure drops to "0". I think I have a fuel pump getting flaky as under WOT, about 6000 rpm the car hesitated, the fuel pressure dropped to about 40psi. Then the fuel pressure jumped up to 50, like the 2ndary fuel pump started. Instant power increase, but I think it was running lean already and the code 55 came up. It definitely feels like its starving for fuel and struggles to get up past 6000 rpm. The car was sitting for 3 months before I went for the injector change so the fuel pump may be failing from sitting idle.

Anyone got any Ideas or am i on the right track?

Thanks in advance