This unit is very old, 20+ years. To run pneumatic tools it still works great. It fills to 115PSI and shut off normally, I blew out 28 irrigation systems the past week as I have done for over 20 years. Each system takes about 20-25 minutes. In the past, my unit would drop to around 80PSI while I ran the pressure at 40PSI for each system. This year, the compressor dropped to around 40PSI while the system ran. It did complete the job and would refill back to full after I shutoff the air valve in a relatively short time. After some reading, I realized that I did not have an air filter or the muffler on the unit. Also I am holding the cover on with duct tape. STOP LAUGHING!!!! My unit is a permanent oil unit and it doesn't make any unusual noises, though it is loud.
Any suggestions? I have til next Fall to get it back to perfect as I don't use it that much. I am just a homeowner and do not use this unit commercially. Thanks to all for your replies.

The unit is a Devilbiss DF430-2. The manual says: MG2-OFTANK-2 dated 6/19/95

Thanks for your reply. The unit does start and stop air as normal. It kicks on at about 100PSI and stops at 115PSI. I, not knowing much about the unit, suspect more that its the compression of the piston but am looking for some more intelligent suggestions.