Dan...My first impression is the pressure switch for your compressor is set at 55PSI. I can understand that because your converter may have set your coach up so the aux. compressor only provides enough pressure for the regulated air system devices such as the floor slide or maybe the driver seat or generator air bags.

Generally a compressor has an over pressure relief valve, and also an unloader valve. The unloader valve will typically spit at the end of a cycle to dump head pressure so the compressor does not start under a load. That should result in a sound of escaping air at the end of the compressor cycle and then stop after a second or two.

The second valve typically found on a compressor (usually one with its own tank) is a pressure relief valve. The purpose of that valve is to provide some high limit safety. For example the compressor tank may be rated for 125 PSI, so the relief valve may start opening and dumping air at 115 PSI. Depending on the type, some reset themselves when the pressure is down safely some predetermined amount, and some require manual resetting.

Most larger compressors have a pressure switch that is adjustable.

As to your problem, my first reaction is your pressure relief valve or the unloader valve is making your compressor cycle. Because the compressor shuts off and then the air flow begins I would look to the unloader.

I would not rule out the pressure relief which may coincidently be set to the exact same pressure as the compressor cut out switch on the pressure regulator.

General thoughts....your aux air system sees full bus air pressure when the engine is running. Do you hear any air escape when the bus has just shut off? If not it may be that you have a check valve between full system pressure and the aux compressor pressure.

Also, some of us have aux compressors built into the bus by the converters that put out sufficient pressure to air up tires and level the coach.

I would suggest you contact Parliament and maybe Rick May in particular to get a better understanding of your coach's system.