benz i am starting to suspect that you are just playing a prank on us,but for sake of me being wrong i will answere your question.
the aux pump produces low pressure to get the fuel to the ip(i think about 10 psi but read if this is important to you), the ip produces high pressure(probalbly about 2500 psi,again read if its important)
the pressure inside the inlet side of the ip is maintained by the fact that the aux pump is working and that the relief valve that is located on the back side of the ip(its screwed directly into the ip pump housing and has a plastic line attached to the fuel filter,it must not be open until it has about 7 psi(again read if its important)take a light and shine in between the ip and the block and you can see.
also would you state your compression values as you have done all that i earlier wrote about?
71 old cars