when to pull the plug??

At one time they were made by Coleman power-mate. Not sure what the deal is now.
You could buy the cylinder and piston kit for them. The piston ring looks like it is made of leather. Dust and such just chews them up. Also they will break one of the reed valves on them causing the slow fill issue. They use an O-ring to seal the head so it wouldn't hurt anything to tear into it and see if you can spot the easy fix.