There is a check valve at the bottom of the one gallon tank, the spring is corroded and collapsed. air is passing the valve and getting to the top of the piston head causing the piston not to move. sometimes you can reseat the valve seal if the spring is good by putting the seal metal rod in a small drill and turning it
on the seat a dozen turns this cleans the seat a makes it seat better. the third fix is a bad reset switch. some times a bad capacitor is the problem.try I known good one . good luck , lee