Bostich 6 Gal Compressor not building pressure

[paredown]

A new thing for me. This compressor has had infrequent use although it is more than ten years old--I have mostly lugged my Porter Cable pancake around (marginally lighter).

I did a day's work with the Bostich--intermittent nailing--after not using it for months. Next day--it would not build pressure, and air seemed to be leaking from under the shroud. I didn't spend a lot of time on it, but took off the shroud, checked all the fittings (nothing seemed untoward). I tried it with the shroud off--and it worked fine.

Next day--back to the no pressure routine..., stuck it in the corner and thought 'later'

So I'm looking for the 411 of compressor repair--what is likely to be failing, what to test etc.

 

[Citation]

Here is a parts manual for what I guess you have. Even if this isn't the correct one, many of these oil free compressors are similar.
https://www.toolpartsdirect.com/bostitch-btfp02012-6-gallon-pancake-compressor.html
If you have put a lot of hours or long continuous runs on the pump it could be the piston ring is going bad (see part 138 in the above link). Another failure on similar compressors is the output hose. It often has a rubber section (#120) that cracks over time. The rubber part gets cracked over time. Finally, a valve could be going bad (valve plate, #108). That would result in slow filling but no leaks after the pump turns off.

If you can, start by removing the shrouds and try to figure out where the air is leaking out. Is the tank holding air or just pressure isn't building? If the tank is leaking out you could have a bad check valve. A worn piston seal wouldn't explain air leaking out while the pump was off. It would only address slow/no pressure build when on.

 

[paredown]

Thanks for the comments--I poked around a little more online after posting this--and it looks like there is a known problem with these compressors with leaking valve plate assemblies--and I found a video on this compressor site.

Failing to build pressure in the tank, and air leaking out from the intake portion of the shroud seems to match what I was seeing. I did check the hose like you mentioned, but my model seems to have updated it to a continuous metal one--and I could feel no leaks anywhere on the hose or connections.

Now I just need to take the shroud off--and see if I can get it to leak while I can see what is happening...

https://aircompressorpartsonline.com/how-to-install-valve-plate-direct-drive-ab9429999-p-72109.html