Problem with fuel pressure

[Andy 4.2]

Fuel pressure drops when car is turned off. I replaced fuel pressure regulator which is genuine Bosch from audi dealer. Anybody knows if there are any other part that can be responsible for fuel pressure beside fuel pump. When car is running, there are no signs of anything being wrong or not working properly. Does the fuel pump should start every time you restart the car or only after a certain time period.
Thanks

[Audi_Tech]

There's a check valve in the fuel pump to keep the supply line from bleeding down and causing extended cranking.
The fuel pump will run all of the time with the engine running, excessive fuel is dumped back into the tank.

[Andy 4.2]

Can I change check valve in the fuel pump without changing the fuel pump itself.
Also, yes I know that the fuel pump working all the time while engine is working however when you restart the car before actually starting the car should fuel pump build up the pressure before?
Thanks

[Audi_Tech]

There should be an approximate 1-2 second running of the pump to prime the system every key cycle.

[Andy 4.2]

Is there any relay that can be stuck, because my pomp dose not re build pressure only after like 2hr, on a half turn, which makes car crank for 30 sec before starting. However it starts fine after few hrs, and pump is working fine.
Thanks