Hope this helps you some.
Worn valves mean increased leakage, and thus poorer air pumping performance.
You set up your compressor to pump into its receiver only, drop the receiver pressure to say 20 psi, then time how long the machine takes to return the pressure to say 100 psi. After repair, time was 54 secs or so again. If your compressor doesn't have its own receiver, you may be able to use the whole plant system as the receiver. I first used this in some power plants here in Victoria, Australia over 20 years ago. Just last year, I read that this test is usual on Navy ships, on their high pressure air systems.