leaking unloader valve

Hi, The unloading system on those smaller piston type compressors, almost always constsist of 2 valves: 1 unloader valve and a check valve. Sometimes they are integrated in the same housing (looks like 1 part), but not always. When the compressor stops, the unloader valve blows off all the air in the discharge pipe of the air compressor (this is done so that the compressor is able to start-up again). Only the discharge pipe should be blown-off. For this reason, there is a check valve installed between this pipe and the tank. Most of the time, this check valve is located right at the point where the discharge pipe enters the tank. It is screwed into the tank. When this check valve is leaking, the whole air tank will slowly blow-off, until the compressor starts up again. More information about the unloader can be found here. You did mention you found a check valve and tested it by blowing air through it. Although that is a goo basic test, it won't tell you if the check valve is completely air-tight. Best is to replace the seals of this check valve, or replace is altogether, because this sound like 100% a check-valve issue. Good luck!