04westernstar air pressure gauge

A valve out? What valve? I would be more inclined to think a bad gauge or sending unit. Not saying it couldn't be a valve, I just don't understand what valve. I also have a 94 WS 4964EX. Love it. I made a bobtail dump out of it 11 years ago. Helluva truck.

 

Problem is your truck most likely has the multiplex gauge systems which means its a lot harder to fix when something gos wrong. It could be the gauge, wiring, communication box, signal line...

 

It's more than likely your gauge. The good thing is, it is very easy to replace and shouldn't be very expensive.
However just for peace of mind you might check this:
Make sure your low air buzzer is working, should come on at about 80 psi.
When the low pressure buzzer is not on, you have more than 40 psi.
If you have an analog gauge there is a small air line on the back of the gauge; when you unplug it you should have air blowing. If you don't have air coming out you have an airing problem. If you have a digital gauge it reads frequency generated by the RX Module. The RX module converts the air pressure to frequency. There should be a 1/4 inch line going to the module(located under the Lh cab leg). If there is no air on it again you have an airing issue.
Hint : you might have to drain the A system in order to unplug the line at the RX module
There is no valve to control the gauge.