The Stovebolt Site Forums The Parking Lot Big Bolts AIR BRAKE PROBLEM

I am thankful for all the great advice and information that people are willing to share on this site. I've been helped very
much. But could I now impose again with a problem?
I have a 65 big chevy with air brakes. It was fine when I parked it a year ago, but now when I start it up, I lose all the
air down to about 30 lbs when I flip the lever on the dash to unlock the trailer. It sounds like the air is being dumped
under the hood somewhere. I think maybe from the valve system that the brake pedal is hooked to. There is no way that the
pump can keep up with this loss of air. We did have freezing weather here in NM last winter.
With the glad hands unhooked, when I flip the lever the air is dumped through just one of them. I have looked and looked
but can't find a hose with a leak. The tractor's brakes are fine when the dash lever is in the locked position. Mert

Hy gto57, when the leak occurs do you have the glad hands connected to a trailer? When you say "unlock the trailer", do you mean release the trailer brakes? It sounds to me like possibly your tractor protection valve is stuck

Compressed air has water vapor in it which condenses in piping. Add freezing weather and you get ice and then broken castings.

Something is broke and letting air out.

Dan Bentler

Yes, the gladhands are connected when I lose the air. I mean by unlocking the trailer brakes, putting air to them by the flipping the switch on the dash. When that switch is to the right, you can't move the trailer. When you move it to the left you should be able to move the trailer but instead all the air is "dumped". Thanks for responding. Mert

Thanks much. Mert

There may be a bleed off line from your dash valve that vents under the hood, if so and its leaking then the valve needs to be repaired or replaced. You'll need to find out what or where the air is coming from.

If your trailer (any modern trailer) has dual chamber spring brakes, you might have a bad brake pod. When you supply air to release the brakes (compressing the brake pod spring), the air you hear may be escaping from the pressurized 'release' chamber to the non-pressurized service brake chamber via a bad rod seal. This non-pressurized chamber is vented through the brake valve so that is why you would hear air escaping from this valve.

When the system is operating normally, there is air holding the spring in the compressed position. When you step on the brake, air fills the service brake chamber, which pushes the rod hooked to the slack adjuster and applies the brakes. When you let off the brake pedal, the brake valve dumps the air pressurized in the service brakes, which allows the rod to retract and release the brakes.

To find the bad brake pod (if this is the case), pinch off the brake release hoses going to each brake pod. Supply air to the trailer and make sure there is no air escaping from the brake valve vent port. Unpinch the brake hoses one at a time, when the air starts escaping from the brake valve, that is the bad brake pod.

Hope this helps!

One quick way to check to see if it is the truck or the trailer is to un hook the blue air line and release the trailer brakes. If air is coming out of the trailer side of the blue line, the problem is in the trailer.

Your tractor protection valve has a quick release valve in it that might be hung. The tractor protection valve has a small air line from the lever on the dash. It also has 4 larger air lines, 2 of which go to the trailer. Find the valve, and have someone flip the toggle switch and see if air comes out of the valve. If its bad, it doesn't necessarily have to be GM. As far as I know, all trucks with the toggle type switch used the same tractor protection valve.

If you haven't found the problem yet a good way to look for the leak would be to hook up your shop air compressor to the truck with the trailer brake valve on (so the air leaks). Without the truck running you should be able to find where the air is leaking out easily as you can hear it a lot better. You can't fix it until you know what is leaking or from where. It could be as simple as a broken air line.

Tad