[77newell]

Floridian: I think your gauge is just fine. As pressure builds in the "supply" tank, starting from zero or some other low pressure, the parts of the air system that don't have anything to do with braking are shut off from the "service" tank by a valve called a "pressure protection valve", aka PPV. This valve is shut any time the "supply" tank is below 60psi in your cas (about 70psi on my coach). This valve minimizes the chances of the brakes being applies due to a large leak in the non-brake air systems dropping the pressure in the brake system to the point your yellow brake knob pops out and applies brakes.

If no compressr is adding air, eventually even small leaks will drop the air pressure to the point the PPV closes. When this happens even a small leak in the non-brake systems will quickly drop the pressure you see on the "service" gauge since it is tied in downstream from the PPV. Now, when compressed air is then being added to the system, assuming starting from pressure below the PPV setting you will see the brake pressure indicators rise, but the "service" gauge will stay at zero until the PPV hits it's set point and opens, allowing air to flow to your non-brake components including the "service" gauge and the pressure indication will suddenly rise.

Hopefully I have not muddied the waters. It's a very simple system. Confusingly, the "service" gauge does not read the pressure in the service tank, but rather reports the pressure available to do work in everything that isn't part of the braking system.