[Johnnyboy]

    Need help, anyone know what this valve does, and should it have a hose hooked up to the bottom barb.

I noticed it was leaking air when I turned key on and  my dash heating controls, what was weird was my supply tank was empty until I turned on heater controls, once on the supply tank filled with 65 lbs of air and I noticed My left side airbags deflated. I assume the airbags filled my supply tank. Any thoughts. Thanks

[HoosierDaddy]

It might be the vacuum generator. It uses air passing over a Venturi to generate vacuum for your hvac control. Tom replaced his with an electric vacuum pump

[Floridian]

Not sure if it helps but my gauge for the SUPPLY air pressure only starts working at 60 PSI, from 0-60 you don't see any movement on the gauge, not sure if it is by design or if I have a leak but it has been always like that (from PO).

Probably you noticed it when you turned on the heating controls but it might have been just a coincidence.

[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.

[Floridian]

that makes a lot more senses than the gauge just reads starting at 60 PSI (on my coach at least) thank you very much for taking the time to explain it. I've always wondered why the brake gauge will work fine but not the supply one, now I know  not sure if everybody is like this with their coach but I love to know every little detail I can on why things work they way they do, it is fascinating to me.

It helps a lot to know this systems because before you get to know them they are like black voodoo magic stuff but after you tinker a little bit with it is not that difficult, at least on my coach, I've read than on newer coaches things are a "million times" more complicated   (just teasing you Tom!)

[77newell]

Yes, all our coaches are plumbed with a PPV for safety reasons. The more non-brake air system stuff we have on our coaches the more places there are for leaks to take control of our brakes unless we have a PPV.