If your air compressor fills up to pressure but struggles to restart or the unloader valve on the pressure switch continues to release air, your check valve is likely the cause. You can run a quick test by running the compressor up to full pressure, once it shuts down disconnect the power to the unit. Loosen the nut on the air exhaust tube at the pump head...there should be no air present. If you have air present, the check valve is bad and needs to be replaced.