Air brake question

We have this 83 freightliner we use to haul cattle and some grain with. When you apply the brakes the secondary air pressure gauge will drop quite fast and the other air gauge doesent move much. If you use the brakes much at all the secondary gauge will drop almost completley off and the low air pressure light comes on, but the air button doesnt pop out. The brakes will continue to work and release and then the secondary gauge pressure will come back up after a bit if you don't use the brakes. I might have to take it in to town to have it looked at, just thought I would ask on here if any one knows what might be causing this. Thanks for any help with this.

The two tanks provide air to two different systems on the truck. One usually is the truck supply and the other the trailer.

Things to check:

1) Hold the brakes down and check for leaks. You could have a blown diaphram. It would be a leak out of the brake chamber.

2) The brakes could be way out of adjustment. Then it takes more air to push the brake cams out to the shoes.

3) One of the quick release valves could be bad and open to dump when pressure is applied.

I would build up the air and shut the truck off. Then press the brakes down and hold them down. You should be able to hear any major leaks.

WIthit stopped, listen for leaks on and around the system. Leaks as serious as the one described are easily detected. Soapy water and a spray bottle are handy as well. Jim

Unless you are absolutely sure you know what you are doing, do not take a brake chamber apart. People have been hurt and killed with them.

Something is wrong the air pressure should only drop back about 15 to 20 pounds for a normal stop. I would test as the others have suggested by also try disconnecting the trailer air (use all necessary safety measures).

Depending on the exact plumbing on your truck there are a couple of things that could be wrong. I know there is typically some sort of 'protection' valve that seperates the primary and secondary side in case of a line rupture, etc. What type your system uses, where it is, etc I can't say for sure but it sounds to me like the valve has shifted and is allowing only the secondary side to be used.

Often these valves are designed as shuttle valves that allow air to get by to keep the secondary chamber charged and it shifts to act as a check valve to block the primary side when it senses a drop in pressure on that side of the valve. The restriction caused by it being stuck in it's shifted position would explain the air being applied from the secondary side only, along with what I gather you were talking about it taking 'a little bit' ((((I read that as longer than it should))) for the secondary side to come back to full pressure.

Here is a link to a Bendix air brake manual that will show you pretty much all you need to know about air brake systems, how they work, and how all the various valve types are used.

I know there is alot more to these systems than I have talked about here. Unfortunately there also many different system designs so a bare bones, generic, idea of the problem is the best I can do here. I just hope this helps you some. Good luck. Wayne
Bendix air brake manual

Fix it before the DOT finds you.

Blown brake chamber, easy fix. Just replace the whole unit.

Have you drained all the air tanks? Ive seen a buildup of water in tanks before that takes away a lot of air capacity.

If someone is outside the truck walking around it he will hear the leak if it's going down that fast. Jim

If you have the red valve pulled out (trailer brakes locked) does it do this?????

The secondary supplies air to the front axle and the trailer.
If it leaks air all the time (red valve in or out) the problem is most likely in the pedal valve; spring brake valve or the front axle chambers.
If it does this only when the red valve is in the problem is most likely in the trailer.

You should be able to hear air leaking from this size leak.

If you find one of the brake chambers is the cause replace the whole thing. Rebuilding by people that do not know how it works is a death sentence.

I usually empty the tanks a few times a year, and there is hardly any water in any of the five tanks. It is the truck, thats the problem, acts the same when the trailer is hooked up or running bobtail. The system holds air until pedal is applied and then the secondary will drop. I tried this with the engine off and really didnt hear any leaking, but the wind was blowing and I was by myself so I couldnt crawl under the truck and listen. Brake pads are thick, so I don't think they are out of adjustment. I am familiar with the spring loaded units and was planning changing a bad one as a unit, I just don't think that is the problem this time. We had a trailer one go bad last year and it had a definate leaking air sound, I just don't hear anything like that on the truck. The stuck valve Wayne describes seems to fit whats going on. The Bendix link has a lot of good information, I didnt take the time to read through it all, antoher time I will. Got some weather moving in so this will have to get set aside for now, won't need it for a bit. I appreciate all the replies and information.

Pad thickness has very, very little to do with adjustment. The slack adjusters are what need to be adjusted properly. Brand new pads can get out of adjustment real fast when you have a driver that doesn't know how to downshift and just uses the brakes to slow down and stop.