DeadmeatXJ

So I recently had my intake/exhaust torn apart and decided to check out fuel pressure. My XJ is a 1993 4.0, could someone explain what I should see in the way of fuel pressure?
When I turn the ignition to on gauge reads zero, starts after a couple cranks and reads 39psi steady, turn engine off and pressure drops to zero in a few seconds. I think I may have a leak somewhere (injector or regulator?). How would recommend I go about narrowing down the culprit?

Thanks.