fuel pressure

sometimes after sitting for a while my car cranks for a while before it starts, so i got to thinking maybe my fuel pressure is bleeding off slowly. So i went to a buddy's shop and put his fuel pressure gauge on, and it was at about 34psi at idle, when i shut it off it went up to about 40 and then slowly went down. i figured it would be normal for it to go back down to the running pressure of 34 or 35 but after about 30 minutes it was at about 26. My question is, how do i tell if it's a fuel pump check valve problem, or a bad regulator, or a leaking injector? the car is a '90 with 117,000 miles on it and i'm sure the pump is original. It IS pretty noisy but it's been like that since i've had the car (3 years). My buddy has an '89 SC and he replaced his pump simply because he thought it was too noisy , and now you can barely hear is even with almost no gas in the tank. Mine is much noisier even with a full tank. So should i just replace the pump first and go from there? I'm assuming it should hold pressure indefinitely, not leak down after 20 or 25 minutes......is this correct?

Thanks!