New truck and a stuck brake! M923

[Artisan]

What happens when you do a COLA test?

 

[RobM923]

I get air out of all 4 valves on the PS, I also get air out of the rear right glad hand, not sure about the pressure but it comes out strong and there is plenty of volume. All of the glad hands are valved.

 

[hemijosh426]

I read somewhere that the ABS is only used on the left side of the truck, it will unlock the left side in the event of a skid, and it has a valve before the brake chamber to control it. Could the ABS valve be not letting air pass thru it?
http://test.steelsoldiers.com/upload/939/M939_ABS_maint_sustainment_tng_std_handout.pdf

 

[wheelspinner]

The knob on the dash is an "EMERGENCY SYSTEM". It works like this: If the truck has a catastrophic air loss which causes the spring brakes to lock in a bad place, for instance on RR tracks, the knob is depressed, releasing a seperate, distinct source of air to only "RELEASE" the spring brkaes, nothing else. In this case the truck could be moved and the application of the brakes will again lock the spring brakes until an air source is provided. That is the ONLY purpose for that knob on the dash. The manual parking brake lever engages the manual drum brake on the drive shaft, and releases the spring brakes.

 

[RobM923]

I have to correct my previous answer, I just bled the truck air system and noticed that very little air if any comes out of the lower bleed valve PS.

 

[RobM923]

haven't done one.

 

[RobM923]

The knob on the dash is an "EMERGENCY SYSTEM". It works like this: If the truck has a catastrophic air loss which causes the spring brakes to lock in a bad place, for instance on RR tracks, the knob is depressed, releasing a seperate, distinct source of air to only "RELEASE" the spring brkaes, nothing else. In this case the truck could be moved and the application of the brakes will again lock the spring brakes until an air source is provided. That is the ONLY purpose for that knob on the dash. The manual parking brake lever engages the manual drum brake on the drive shaft, and releases the spring brakes.

Thanks for the clear explanation.

 

[hotbox]

Mine, on occasion do the same thing. I just bump it in reverse and it releases. It is probably from sitting in formation for years.

 

[jarhead1086]

I am currently wrestling the same type of issues. I recently recovered my 923 from Barstow to Vegas with many issues, but the biggest has been the brakes. My passenger rears were locking up trying to send me to the ditch. As RobM923 stated I would let them cool and they would release so I could roll in neutral. I caged them per the -10 TM with some success and it got me one exit at a time to North Vegas. I bailed home in my chase car and left the truck at a Fleet shop. They found two air lines crossed. They put the adjusters at the right gap to and commented that all the parts looked brand new. Spring cans were green on a tan truck. I was lucky to find a Vegas SS member to pick it up for me. He made it most the way to Henderson and the drivers side locked up on him. The next day he fiddled with the parking handle and got it home. I know the parking brake handle is out of adjustment, so per SW1 Bradley's advice I will adjust per the TM's before I throw new parts at it. A good lesson is to bring a Smartphone or print out TM's if you are a cheap flip phone guy like me. I am worried about the grease in the hubs and will check that before I head north to Utah. I will also try the reverse bump thing per Hotbox's suggestion.

 

[rrrr]

I had the same issue with my 932. Luckily I had it trailered to my home. Took me 3 days and the help of a few members here on the forum to locate and switch the two air lines. What a pain in the a...............

 

[RobM923]

I had the same issue with my 932. Luckily I had it trailered to my home. Took me 3 days and the help of a few members here on the forum to locate and switch the two air lines. What a pain in the a...............

were the crossed lines at the valve bodies were the line converge above the axles? how did you troubleshoot the problem? any help would be great

 

[rrrr]

two lines on one rear wheel were crossed. I looked at the other rear wheels and traced them from the actuator to the valve assembly to see if any were different. One rear tire was.

 

[PETE BALLARD]

To all - to keep your health and limbs safe you should read the TM's and get real familiar with the system and components before trying to repair these items. Also what type of brakes are you working on "s" cam or wedge style - both have good and bad points. The wedge style stop goog if you use your vehicle every day. personaally i prefer the "s" cam style. the wege type is famous for locking up a wheel just after application. for those of you you like to fix everything the emergency brake chamber is one thing best replaced with new even it you are an experience mechanic and they are usually real cheap. sorry did not mean to get on the soap box!!

 

[rrrr]

I agree. In my search to fix my issue I replaced all the rear diaphrams. The spring tension on this is great. One of my caging bolts even broke, scared the crap out of me. I spent huge amounts of time going through the TM's and the forum to become familiar with the trucks systems. It took 3 days and the help of another forum member and mulitple phone calls to figure out the simple fix.

 

[PETE BALLARD]

I read somewhere that the ABS is only used on the left side of the truck, it will unlock the left side in the event of a skid, and it has a valve before the brake chamber to control it. Could the ABS valve be not letting air pass thru it?
http://test.steelsoldiers.com/upload/939/M939_ABS_maint_sustainment_tng_std_handout.pdf

the abs system is always installed to operational on both sides, yes, one of the service valves could get stuck and not release the service brake application air pressue, anything is possible but with wewdge brakes my thoughts are stuck hardware inside the wheel end and also do not rule out the lining beinf frozen to the drum especially if they have metalic lining. also the ABS sytem has nothing to do operatioally with your emergency parking brake operation

 

[rrrr]

The abs is on the rear most axle only and does not sound like your problem. I would loose air when my parking brake was off. When I put on the brake the air loss would stop.

Can you identify what particular wheel/wheels are locked. Start with those and trouble shoot the air lines. You might need to pull the wheels and hubs to check the brakes out.

 

[Csm Davis]

Hi guys, I am new to this site as well as the M923 I just bought. On my drive home my chase truck called and told me the rear axle was smoking. Upon inspection I found the right rear wheel's brake was stuck. I couldn't find anything obviously wrong and it seemed to stick after a somewhat hard brake to make a left turn, so I don't think it was like that at the beginning of the trip. I ended up caging the brakes for that wheel and driving her home to deal with it there.

The ABS light is always on, and the air system seems to be working correctly. All of the other wheels seem alright, and the primary and secondary tanks work the range from 90-120 depending on braking frequency. I drained the tanks when I got home and didn't see any water escaping that may have made it past the drier.

Any suggestions on where to start? Any help is appreciated. I am not entirely new to the 121 air systems, but it has been a couple years since I have even been near one, and have never worked on anything military except my M1009, which is pretty basic.

Okay did you ever find out your problem? Sounds like bad diaphragm, maybe crossed lines but not likely.

 

[Csm Davis]

Both tanks pressure up they read somewhere between 110 and 120 psi (never took note exactly how many psi). Yesterday I noticed the spring brakes on the center axle are caged, not sure why. Today I plan on uncaging them and continuing to troubleshoot the brakes. Still havent found why my rear brakes (both axles) won't release when I take my foot off the brake pedal

This probably points to a problem with your system not just one axle. Also you said all of your gladhands are valved? Front passenger gladhand should Not be closed off. It should have a cover with a vent on it.

 

[wheelspinner]

The abs is on the rear most axle only and does not sound like your problem. I would loose air when my parking brake was off. When I put on the brake the air loss would stop.

Where did you get this information? All of the info I have read including the Army Supplement issued specifically for the ABS MRO states that the rear "axles" are equipped.

 

[rrrr]

The abs manual talks about typical installation.. Look at your rear axles, you'll only see abs sensors on the rear most axle.