Fuel pressure readings what do they mean?

[jsd3024]

I checked the pressure from WUR to fuel distributor and here are the numbers. Cold start reading was 20psi and within 15-20 seconds climbs to 50 psi. It held at 50 as the engine warmed and stayed their until engine was shut off. Pressure drops to 0 as soon as the engine is shut off. I tried it both with the shut off valve open and closed on the gauge and went to zero both ways. I don't think that's correct but I need help diagnosing it. Any ideas?