I have a bostitch 6 gal pancake compressor that won't build pressure past about 20 lbs. when I turn it off I don't hear air leaking out anywhere and when it's running it runs at a higher pitch ( similar to a shop vac when it gets clogged) I was using it to spray wall texture so it was running pretty constant for about 20 min. before the problem occured. It was working fine when I started. had full presure 120 lbs. and I was running the outlet pressure at 60 lbs
Air Tools & Compressors - Stanley - Bostitch CAP2060P 105 Amp 2Horsepower 6Gallon OilFree...
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Http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/bostitch/CAP2060P/
http://d3cmirsdb60x3h.cloudfront.net/schematics/bostitch/cap2060p.big.pdf
Check the valve in the head. Look for a bad gasket, especially the one that goes between the cylinder head and the valve assembly. A piece of something might be jammed in a reed valve.
Good luck and DON'T overtighten the head bolts when you reassemble your machine.
I'd pull the cover off to expose top of cylinder head and have a look at any intake air filter. In parts diagram, It appears there is a small one that would likely clog during the texturing process.