Tighten the air compressor's air tube fittings where air escapes and replace any damaged air tubes. The check valve prevents tank air from back flowing to the pump and unloader. If air leaks out the unloader when the pump stops, replace the check valv,e because the check valve leaks air back towards the unloader.
The safety valve pops open to relieve excessive air pressure if the pressure switch fails to shut off the air compressor at the cut-out pressure setting. Replace the safety valve if it leaks air constantly. Replace the air compressor tank if it leaks air.
Rebuild the air compressor pump if it leaks air through the pump head when the pump runs because a broken valve plate seal is allowing air to escape through the pump head.