M931 a1 Rear Brakes Locking Up

[Hollndr]

Hello all, been searching the technical manual and through threads for a solution to my issue but have yet to find it, if I missed it please point me to the right direction. Sorry if this has been covered, i'm new to military trucks.

I Have an '85 M931 A1 with ABS, about 1200 miles since 2010 gov rebuild. Recently had an issue where my rear brakes locked up on me and would not release. I was backing up to a trailer (pintle hookup). Upon hooking up and driving away the truck felt bogged down. Unhooked the trailer and pulled forward a little and truck completely locked to where it wouldnt move. Messed with the parking brake in and out as well as the maunal brake switch on the dash. Air pressure for primary and secondary tanks were both over 120psi with no drop in pressure over time. Had to have a tow truck come and they were able to get the brakes to release by messing with the parking brake switch and manual brake button on dash. Tow truck owner told me the parking brake switch sets both the parking brake on the driveshaft as well as the drum brakes, but was not releasing the drum brakes when the parking brake was released. Said I may need to replace the parking brake switch, could be going bad. Drove probably another 100 miles or so with no issue until I went to hook up trailer again and brakes locked up again. Messed with both parking brake and manual brake, draining and re-pressurizing air system and couldnt get it to release. Walked away for a couple hours and when I came back the brakes had released and I could move it again. Went to drive the truck and seemed to drive ok but felt a little under powered. About 2 miles from my house at about 55mph my low air alarm / light went off (air pressure on guages both over 120psi). pulled over, backed up (alarm went off) and drove back home. drums felt a little warm for the little braking I did.

Im uncertain as to what may be happening here and wondering if anyone has had this issue and your fix (is it the parking brake switch?) or if anyone whos more knowlegable than me may be able to point me in the right direction. Thanks in advance for any of your help!

 

[porkysplace]

Check your front gladhand .
[h=1]M925a2 brakes are staying engaged[/h]

 

[juanprado]

Lots of tractors sold out of camp Shelby that sat up had rust issues of brake shoes stuck to drums. Common for drums that needed a big hammer to release shoes. You can also pull dust covers off backing plates to look at shoes if seized on drums?

 

[Floridianson]

Are you setting the parking brakes when hooking up and then they are not coming off? Do believe the glad hand thing is the last catch if something is going wrong in the system so check that. Then I would check the roller valve on the parking brake. When that is down it should be venting out the intake as the parking brakes come off. Air up the system and shut the truck down and try by hand working that roller valve. If it is hanging up you could add air tool oil to the line that that feeds it. The feed line is the line closes to the back of the cab. If all the brakes are hanging up it would be in the treadle valve or as said the glad hand. If just the parking brake then something else as the service line is a different circuit from the parking brake circuit.

 

[Hollndr]

My service gladhand (pass side) does not have a valve and i believe it is a vented type. I grabbed some air tool oil from work and will try that tomorrow night.

I did pull the brake dust covers and it looked good from what i could see.

This is a recent issue, driven the truck for a good 800 miles and now just having the issue. Yes i did set the parking brake to hook up the trailer. My air vents every time parking brake is applied. Ive also tried operating the roller previously by hand and still no release.

Hopefully adding some oil to the lines will free things up tomorrow. Thanks to all for your help and advice.

 

[Floridianson]

My post reads funny. If all the service brakes are hanging up then check glad hand. If just the parking brakes then the roller/ parking brake circuit and no air leaks.

 

[Hollndr]

Finally got a little time on the truck yesterday. Looks like i dont have the vented gladhand in the front. No hole on inside of gladhand, appears to fully seal when in place. Ill look into getting a vented.

I guess my question on this would be why all of a sudden would a non vented gladhand be an issue after driving for 800 miles no issue?

Also where would be a good place in the system to add the air oil?

Thanks again for all you guys help, sorry for the newb questions.

Take 'er easy

 

[Hollndr]

So i have not been having the brake lockup issue recently however i did install a vented gladhand anyways and threw some air oil in the lines.

I do have a new issue that had been going on since i bought the truck but never showed signs unless at high speeds (60mph) and never occured more than 1-2 seconds. The issue is my low air light is now flickering as i accelerate mainly. I did have a solid low air with alarm once. Pulled over, backed up and alarm went off. Low air light however still flickers now as im accelerating and occasional when cruising over 45mph. Both pressures are 120 and vent off when get over 125psi. Anyone had this issue?

 

[The HUlk]

I'm a newb like you but have learned quite a bit from this site and digging into the manuals.

Moist air will cause sticky air valves & air switches which will manifest itself in quirky issues like you are experiencing. It's also a good idea to drain the air system using the four petcock valves behind the cab on the passenger side frequently to blow out moisture collecting in the tanks.

You may consider replacing your air dryer if it is from 2010. It has desiccant material to reduce the amount of moisture in the air delivered by the compressor. You can change the desiccant canister but the valves in the dryers will get sticky and I feel it is better to replace rather than rebuild.

When your low air light comes on you should get an audible alarm from the module on the drivers side footwell on the left side above the outside air vent. If no alarm then the buzzer in the module is bad at a minimum. The low air alarm is normally generated from either of two normally closed air pressure switches wired in series which connect to the module. It is possible one or both of these switches need to be lubed or replaced, or there is an intermittent wiring problem, bad ground, etc. Post #32 of this thread may be helpful...

http://www.steelsoldiers.com/showth...ake-buzzer-inop-but-warning-lights-work/page4

 

[Hollndr]

Checked my air dryer over and that appeared to be ok. Would a bad dryer / dryer valves trigger the alarm if pressure is good?

Followed the technical manual for low air alarm with normal pressure on guages. Switches on primary and secondary tanks both showed below 300 ohms when over 90 psi on system. Also jumpered connections as described to check for bad wires, didnt shut alarm off. Checked pin "f" at fail control module to ground and was well under 600 ohms. Manual points to a bad fail control module however the alarm works properly for parking brake and function. My low air light goes off after pressure is in the system but the alarm is still there. I dont want to go out and buy a new module at $170 if thats not the issue. Any ideas?

I regularly blow my air down after every use.

 

[maverick50]

Just some info guys. I pull doubles which has more air lines then a regular truck which means we have problems all of the time. Put some alcohol into the air lines to dry things up. You can go to a truck stop, they have some special alcohol for the air lines. Try that before going with the air dryer.

 

[The HUlk]

Checked my air dryer over and that appeared to be ok. Would a bad dryer / dryer valves trigger the alarm if pressure is good?

Followed the technical manual for low air alarm with normal pressure on guages. Switches on primary and secondary tanks both showed below 300 ohms when over 90 psi on system. Also jumpered connections as described to check for bad wires, didnt shut alarm off. Checked pin "f" at fail control module to ground and was well under 600 ohms. Manual points to a bad fail control module however the alarm works properly for parking brake and function. My low air light goes off after pressure is in the system but the alarm is still there. I dont want to go out and buy a new module at $170 if thats not the issue. Any ideas?

I regularly blow my air down after every use.

The failsafe box is bad. It is not uncommon to have partial good & bad functionality in circuits such as this. One section of the buzzer circuit has failed. Safety warning circuits are purposefully designed to give a false warning when there is a failure within them. This is so that a failure does not go unnoticed.

The air dyer only has to do with the actual air pressure which is good. Should be no problem there if guages read good and are accurate.

 

[wrenchturner6238]

The air dryer will not trigger your alarm. If the air dryer is bad you will not be able to build air (purge valve sticking and leaking air). With your low air light problem (I have not had this problem yet) It sounds like to me you have a connection problem which could be in your controller. If you go through the manual correctly (step by step) it will lead you correctly. Go through the checks again and see if you come to the same end.

 

[The HUlk]

Electrical engineer here who HAS had problems with the failsafe box and designed the circuit below for those who would like to build their own rather than buy a new one. If there is still a question as to whether the box is bad you can be certain by disconnecting wire 584 from PIN I and disconnecting wire 578 from PIN F. This removes the pressure switches, parking brake switch, and wiring from the equation. In this state there should be a low air light and buzzer. Then apply a GND to PIN F. When the ground is applied the air low lamp should go out and the buzzer deactivate at the same time. When the ground is removed the low air light should come on and the buzzer activate at the same time.

 

[Hollndr]

Thanks for help. I ordered a new failsafe box. Found one for 125. Should be here next week. Ill post the results...

Take er easy

 

[Hollndr]

Well the issue isnt the failsafe module... same issue as old one, air pressure is good, low air light goes off and alarm is still on... keep looking through tm i suppose.

 

[MtnSnow]

parking brake switch

 

[Hollndr]

Determined the issue is in the engine ignition circuit somewhere. This would be the buzzer when battery is on and ignition is on before switching fully to crank engine. The buzzer now never goes off. With wire snipped to failsafe module buzzer is silent and other buzzers in module work as should... to the tech manuals now. Ill let everyone know outcome once i get it nailed down.

Thanks all

 

[The HUlk]

Determined the issue is in the engine ignition circuit somewhere. This would be the buzzer when battery is on and ignition is on before switching fully to crank engine. The buzzer now never goes off. With wire snipped to failsafe module buzzer is silent and other buzzers in module work as should... to the tech manuals now. Ill let everyone know outcome once i get it nailed down.

Thanks all

It's possible that more than just the failsafe module was bad.

When the ignition switch is turned on power is applied to the failsafe box. That is part of the reason the buzzer starts when the ignition is turned on. The buzzer should stay on until the air pressure is good and the parking brake is released. The low air lamp should also go out when the air is good and the parking brake lamp should go out when the brake is released.

Are both lamps lighting when they should and going out when they should?
Which wire stops the buzzer when you disconnect it and which lamp goes out?

 

[Hollndr]

Built air pressure on system to above 120 psi, alarm shuts off at 60 psi as should, dummy light also shuts off. Also troubleshot air system per technical manual with no issues present. Parking brake also shuts off when disengaged. The wire is pin A on the failsafe connector which is the engine ignition circuit. Theres no dummy lamp associated with this buzzer.