Any ideas? It always used to go up to 125 psi and stop. Then, one time I noticed and it was 130 psi while cruising. Now, when I'm hill climbing, I bury the needle as I said. As soon as I stop climbing, it levels off around 135 psi.

How bad is the threat (how much air can the system handle)? What's likely the problem? Cost of repair/parts? TIA

Sounds like the governor on the compressor is taking a dump.

I sure hope the relief valve is working!

I agree, the air pressure governor has failed, or the air line between the wet tank and the governor, or the line between the governor and the compressor (if remotely mounted) is clogged or broken.

I would think you also have a bad relief valve on the compressor. I think they should pop off around 150.

Ok, thanks for the replies. I confess, I don't even know what the regulator looks like, and I didn't know there is a relief valve. The needle was buried, though, must have been somewhere around 170 psi. I see there's a place not too far from my pickup, Diamond State Truck Center in Elkton, MD. I'll give them a try (after I crawl under there and see if I can find anything obvious).

You know that "pop and hiss" that the compressor does when it kicks out? That's the relief valve.

Honestly no, I'll have to listen for it. I've had this intermittent air leak behind the dash, I've been up in there myself before working on it. Anyway, the sound I'm always conscious of is the sound of leaking air behind the dash.

Thinking about it, I guess I do know that sound, but what does the part look like?

If you can hear air bleeding off behind the dash, you likely have an air leak some where on your tractor or trailer(s). This has happened to me. The air leaking out elsewhere creates a hissing or moaning sound near the valves on your dash. It could be leaking behind the dash, but I would guess it is somewhere else.

The pop-off valve or relief valve is generally located on the air dryer. A good service shop should be able to point right to it. With some investigation you can find it yourself, but in most cases, you will need to get under the truck.

as far as air leak behind the dash.. it could be your park brake. the one with the yellow/red knobs. but you cannot detect leak by hearing it. you have to soap it, or put other source of leak detector on it. don't do like I did one time.. I clearly heard air when I put my ear to the dash valve. I replaced the valve. the leak was not in the valve. it was a leaking air bag!!! the air that was rushing to replace the air lost from bag could be heard from dash, but that was not where the leak was.

Truck fixed. Bolt missing on the regulator (they call it a governor), said it was hanging. New governor installed, $125.

I've dealt with that dash leak before, I know exactly where it is. It's intermittent, and though it's too noisy, the rate of air loss isn't too bad. Anyway, unless something changes, I'm not ready to tear the dash apart again.