[Richard]

Reese,

I think we may have some confusion on relief valves.

There is one valve that is at the bottom of the air dryer cannister located behind the driver tag tire. It will pop every time the air compressor on the engine runs through a pressure build cycle. That only happens when the engine is running. Many people will refer to this as the unloader valve. It does two things. It releases the pressure on the engine driven air compressor, and it exhausts any water collected in the bottom of the air dryer when it pops.

There is a small safety valve in the bottom of the wet tank, located underneath the coach, that is intended as a safety device to keep the entire system from seeing too high of a pressure. They are spring loaded and release pressure above a certain point. They can be bought to relieve at different pressures. They look like this https://www.mscdirect.com/browse/tnpla/3...yAQAvD_BwE

I am completely puzzled by your mention of a relief valve on the firewall. Please describe it, or take a picture and post it. I have never seen a Newell with a pressure relief valve on the firewall. I have seen a quarter turn valve on the firewall that is used to supply air to the air doubler. It would release pressure on the system if opened.

So, did you try reducing the pressure on the compressor to 60 psi? What happened?