Because of the plastic lines it is not easy to T in a gauge to check the lift pump pressure. And when you can the needle bounces back and forth so much it is difficult to tell. Some have dealt with that by using liquid dampened gauges and some have used some sort of restricted orifice.
One person used that skinny plastic tubing that people use when they add on a mechanical Oil Pressure gauge and I guess the small inside diameter and the length of the tubing dampened out the impulses.
The Lift Pump is a piston type pump and is positive displacement in one direction and when the spring is compressed the spring tension move the Piston back I think the pressure was high enough it would hold the piston in that position till the pressure bleeds off. The Fuel Pressure Relief/Overflow valve determines the pressure.
Post 5 and post 11 have pictures of the innards I posted in a previous thread.
Can the 617 injection leak into the oil?
Note that if you are removing the big plug on the Lift Pump when you put it into a vice keep away from the thin flanges on the casting as they are easy to crack off.