Fuel pressure dropping over night

2000 S Type V8

I changed the fuel pump, and now the car runs, but it seems fuel pressure is dropping off.

I can cycle the ignition on a few times to get the 1 second priming burst, then it starts pretty fast.

Is there a one way valve in the pump, or in the system that has failed?



By the sounds of it yes, the cycle the ignition switch is the give away.
When I had that on my ‘92 XJ40 I managed to insert a one way valve in the engine compartment which solved the problem.