Service Brakes not Releasing

[M35A2-AZ]

I had a problem with my M923A1 today. The service brakes are not releasing.

I went into town to the post office (about 10 miles) and I noticed that the truck was not getting up the speed. When I got to the Post Office I noticed the rear brake were hot. I let them cool as I tried to find out why they got hot. I looked at the cage brakes they looked ok, but I caged them anyway.

Thinking I had them released I headed home slowly. I stopped at about 3 miles and checked the brakes, they were hot again.
I looked over the truck as may wife pushed on the brakes. Now the brakes are locked up and will not release.

I called a friend on SS "Castle Bravo" and we talked a little and he said to drain the air tanks and see if the brakes release, they did?

So it is looking like the service brakes are not releasing after you push on the brake paddle and the more you push on the brakes the more they push.

Does any one have any ideas on the problem? I will try to start trouble shooting in a day or so.

 

[F18hornetM]

I am not familar with the 923 brakes, but there may be a quick release or relay valve near the rear axle somewhere thats hung up. thats the way it is on commercial trucks.
Caging the brake chambers will only "hold" back the parking brake spring from applying. not the service side as you noticed. By letting all the air out of the tanks, the air eascaped the brake chmabers and the return spring pushed the service diapragms back, releasing the brakes...having said that, and depending on if you just let the air out of the tanks...might be a foot valve issue..thats the part the brake pedal is hooked to..
again this is just general air brake stuff, may not even apply to a 923
If i am not mistaken, are the brakes wedge type??

 

[M35A2-AZ]

F18hornetM, Thanks for the reply and ideas to look at.
I did notice that the front brakes did not get hot.

 

[Nonotagain]

I posted the PDF from Axletech the manufacturer of the wedge brakes.

http://www.steelsoldiers.com/showthread.php?t=45908

Chances are that the two "barrels" that do the spreading of the shoes are rusted up.

 

[M35A2-AZ]

Nonotagain, Thanks!!
But it was on all four rear brakes and not the front.

 

[Nonotagain]

Nonotagain, Thanks!!
But it was on all four rear brakes and not the front.

Try this one then.
Search for brakes and start around page 255

 

[M35A2-AZ]

Went out this morning looked at some things. Started truck, checked brakes
worked ok. Started to drive around our block, got about a 1/2 mile and the brakes locked up again.
I am starting to think it maybe the ABS system.
Is there a way to disable the ABS system?

 

[Derrickl112]

He never posted if it worked or not....but does this help at all?


http://www.steelsoldiers.com/5-ton-up/32783-m936a2-brakes-slowly-lock-up-what-gives.html

 

[rrrr]

It sounds like since its only the 4 rear wheel assemblies something is going on with your spring brakes. Perhaps some air issues ?

I would try caging all 4 rear spring brakes and do a slow test drive in an area that if the brakes stop working you'll have no issues. If the brakes still get hot than its not in your spring assemblies and we can try to help you trouble shoot other areas. Eliminate one system at a time.

My 932 had an issue and it ended up having an air line that was reversed with another air line. It took 3 days of trouble shooting to find that one. The TM's for this are not good either.

 

[emmado22]

Is your air pressure constant, and have you checked it with something OTHER than the guage on the dash? Sounds like an air pressure problem to me. My $$ is on a leak.

 

[73m819]

It sounds like since its only the 4 rear wheel assemblies something is going on with your spring brakes. Perhaps some air issues ?

I would try caging all 4 rear spring brakes and do a slow test drive in an area that if the brakes stop working you'll have no issues. If the brakes still get hot than its not in your spring assemblies and we can try to help you trouble shoot other areas. Eliminate one system at a time.

My 932 had an issue and it ended up having an air line that was reversed with another air line. It took 3 days of trouble shooting to find that one. The TM's for this are not good either.[/QUOTE]

That is a mild statemant, thay ARE WORTHLESS

I'm guessing that if both axles still getting hot after caging, then look for a relay valve or a quick release valve hanging up

 

[emmado22]

https://premiernorth.com/m939/uploads/AMG%20ABS%20MWO%20Field%20Service,%2014%20Feb%2003.pdf


This may help.

 

[emmado22]

https://premiernorth.com/m939/uploads/AMG ABS MWO Field Service, 14 Feb 03.pdf


This may help.

 

[FASTNOVA]

Do the brakes apply on their own (without stepping on the brake pedal) or do they stay applied after you have pressed on the brakes?

 

[Nonotagain]

Page 255 of the TM I posted refers to the rear boogies and the brakes.

It's a relay problem. There are pics of the air tanks and valves, and a small trouble shooting section.

 

[F18hornetM]

Here is a test for a relay valve. With air pressure built up, and truck NOT running. WHEELS CHOCKED!!! Crawl under truck and find relay valve. Have a friend apply the brakes. When they release the pedal, there should be a release of air from the quick release valve. I have seen mud duabers build nests in trailer valves that plug them up. You would think the air escaping would blow it right out, but nope. have to use a pic to clean it out. Sometimes you can take the valve apart and just clean it, today we just replace them.
Again this is generic air brake info..
If it had a large enough air leak, that the spring brakes were applying, and the compressor couldnt keep up, you would hear that leak for sure!!

 

[M35A2-AZ]

Thanks Guys for all the inputs. Have not had much more time to work on it, today is my wifes Birthday.

With the brakes caged, the rear brakes lock up, not the fronts.

I did notice this morning that when the brakes are working the relay valve is letting air out. When the brakes are not working no air is coming out of the relay valve.It looks like there is two relay valves, one forward and one behind that one to the rear, the most rear one is the one letting the air out when working.

Thanks again!!
happy New Year!!

 

[F18hornetM]

One of those relay valves may be for the trailer, not sure. You can follow brake tubing to be sure.
I wish I had one to look at for you. as Im not sure how the foot valve operates on those.
but another thought is, to test the foot valve, is...when locked up...remove the pilot line coming back to the relay valve from the foot valve...there should have no air on it, when brake pedal released...I doubt the foot valve is the issue, the front works fine and as Ive only seen 1 or 2 bad in 25+ years of wokring on trucks. But this would test it.
Just some more info... the relay valve is like a solenoid in an eletrical system...it take a small amount of air and opens a "relay" to allow air pressure from the trucks tanks to operate the brakes...then when released it allows the air to escape quickly instead of travelling all the way back and out the foot valve..Hope some of this makes sense

Happy birthday to your wife

 

[M35A2-AZ]

Do the brakes apply on their own (without stepping on the brake pedal) or do they stay applied after you have pressed on the brakes?

The brakes stay applied after you press on the brakes.
Thanks.

 

[rrrr]

You might have two issues. The release valve and the brake pedal. If your truck has sat for a while and you don't empty the air tanks of moisture you might have some rust on the valves causing them to be sticky. I opened up my emergency glad hand on the very back and squirt some air tool oil in. I then service the air system with an air compressor and a glad hand set up to use an air compressor connection. Once pressured up pump your brakes to try and circulate the oiled air around your trucks air system. I did this twice on my recent GL purchase. Check the brake release vents and make sure they are not clogged as mention earlier. If your truck has abs you will have an air dryer but since you are adding oil at the end of the systme it shouldn't impact your dryer. You might have to do this a few times. An air compressor is great help because you can trouble shoot without the truck running. You can hear the air doing its thing. If you don't know how to set up a glad hand to use on an air compressor post up and I'll take a picture and post it. This was great help when I had issues with my system.

The bottom line is if you pump the brakes and your air pressure doesn't respond much and your brakes stay engaged your outlet vavle is not releasing. If your air pressure is going down then your brake valve is staying in the on, brakes applied, position. Either one of these problems will cause your brakes to stay applied after you release the pedal.

If you want to talk pm me with your number and I'll call you.