Fuel pressure dropping over night

Hello,
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?

Thanks,

Rob

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.