Check fuses first..give them a spin.

The ecu needs a TN speed signal to operate the FP relay..
..but you want to check the FP and filter for flow by themselves ..so, make a jumper wire with a fuse in-line and jumper pins 87 and 30 at the FP relay connector while you have the gauge on the fuel rail..do not run the car, just run the pumps so you can verify that side of the fuel delivery..you should see 3-4 bar pressure and it should hold after you remove the jumper.........
....if still low/no pressure , the problem is FP or filter. If pleanty of pressure, then the relay is not seeing a TN signal and dropping the pump circuit..could be a poor connection at fuse feed to ecu , unplugged cam sensor, or bad OVP feed to ecu...tap the ovp a few times ,, the fuse can look fine , but the OVP can be bad and that will cause no feed to ECU, which in turn means no feed sigto FP relay.
Isolate/verify pumps w/jumper first...

__________________
A Dalton