I got one of these from a buddy and was told it would run but not build any pressure. He "gave" me the compressor figuring it was a boat anchor (useless) and I took apart each area of the compressor from the airline receiver back to the compressor. Eventually I landed in the compressor's pump piston and the intake head where I discovered that the valve system used had three (3) small stainless steel "flaps" that were supposed to function to open and close the intake port and compression port as the piston traveled up and down. The "center" flap had snapped off where it was mounted on the head which explained why there was no build up of pressure, it escaped as quickly as it was drawn into the cylinder through the same port due to the broken off flap not being there to close the intake port.
After a search on the web about the issue, I found these compressors had a serious issue with the valve system and found tons of references to other owners with the same issue, "runs but won't build up pressure". Worse yet, in my search, I discovered that simply "replacing" the broken valve with the "Bostitch" part, they would break again in relativly fast succession and one owner said he replaced his three times before he just put the compressor under a work bench and wrote it off.
Apparently, the "stress" and viabration of it's function is too much for the designed "single valve" as it was reffered to) system and these little "flaps" were prone to "snap off" in a very short period of use and even after replacement did not hold up. Incidentally, I also noted rust pitting on the flaps from their exposure to humidity during use and even if the "stress and viabration" didn't break them, it was apparent that it would not take long for the rust to begin effecting the compressor's ability to develope pressure through leakage from the pitting holes.
The "Good News" is, after a search I happened upon a company that offered a "refit" valve and head assembly that replaces the "single valve" system with a "redesigned" "Two Valve" system that bolts right in ! I called the company and asked if this was specifally designed for my application and was told it was so, I ordered it.
I didn't order the new "piston ring" as mine did not seem to have gotten "torn up" by any of the parts that had broken, you will want to check yours before ordering because if the "ring" is damaged why not get the whole problem solved at once? Anyway, my parts came very fast (I think 2-3 days from my order date), I followed the included diagram (be sure the valve plate is facing the right direction (valve ports towards the "back") assembled the rest and WaLa !! She pumps like new !
Needless to say my "generous" friend was upset, the total with shipping was $32.00 and the repair (not inclding the waiting period for parts) was about an hour and a half !
Here is the company website and part number I hope will help you solve your problem, good luck!
(0006) VALVE PLATE RETRO KIT Part #AB-9429999