I’ve had an intermittant air leak at my Tu-Flo 750 air compressor. This is the compressor attached to the engine used for brakes, suspension, etc. The leak finally became constant. Turns out my Discharge Line Safety Valve had gone bad. Normally the Discharge Line Safety Valve only releases air when the pressure exceeds 250psi. Mine began to leak at almost any pressure. Unit had gone bad. Apparently this is common. The valve is screwed into the head assembly of the comprssor between the compressor and the main engine S60. It is possiblt to R & R the valve without removing the compressor. I used an 8″ crescent wrench to do the trick. A few choice words and 1 hour will get the job done. Assuming you do not have too much convertor equipment in the way.