Most APM failures are due to carbon build up - If we fit a compressor we have to clean out the pipes from the compressor to the APM - Sometimes it end up blocking them though - The stuffs pretty potent as it takes the coating off the inside of the flex pipe so that has to be replaced afterwards no matter what. Always change the silencer on the bottom of the APM when changing its filter as this clogs as well causing back pressure. If you get a genuine filter you alsoget a yellow sieve filter that goes inside the APM . Your problem could be either the APM is jamming and its leaking all the time but you can’t hear it very well as its coming out of the silencer or the compressor valves are sticking. There is a pipe from the APM back to the compressor that feeds air at max pressure to let the compressor run light - disconnecting this pipe can stop this but then the compressor runs heavy all the time.
Steve.