M931 a1 Rear Brakes Locking Up

[JohnnyBM931A2]

I haven't had an issue on my m931a2. How long does your truck sit between uses? Curious if I've just been lucky, or if I move my truck more.

Mine sits in the open and we had a lot of rain over last fall, but our summers are very dry.

Mine locked up after about 2 months of sitting. Now I drive it regularly and it hasn't happened again.

 

[JohnnyBM931A2]

So once again my M931/SEMTT had a locked up rear brake. This time it wasn't a simple backup and it unlocked. I had to get onto it with a sledgehammer. But it got me to thinking. I *think* most of the problems with locking up rear brakes happen with the super singled tractors correct? This due to the fact that the drums are exposed to rain easily. So I'm thinking about making a simple "rain shield" for the rear drums. Just a piece of sheetmetal with two tabs sticking out bent at a 90*, then curve the larger area to match the profile of the drum. I'm thinking some short self tapping screws could be used to mount it to the drums. Anyone think this might be an idea worth pursuing? I'll mock up something this afternoon with cardboard to show what I'm thinking.

I really like your idea However, wouldn't it just be simpler to cover the rear of the truck with a large tarp when it's not in use?

 

[doghead]

Trailers do this also and they are well covered.

 

[Csm Davis]

This is not a rain problem but condensation and it happens because the brakes are applied when trucks sit it will happen with any drum brake that is left engaged for long periods of time generally in high humidity areas but can happen any where you get a dew fall.

 

[Csm Davis]

That isn't a bad idea at all. Some grease in the proper place would help too, but tearing each wheel end down just to apply a little bit of grease is off putting at best.

The grease on the wedge is a good thing but most not all are hung on the pads not the wedge that's why the hammer to the drum works, when the wedge sticks usually a good tap on the tube that holds the spring and actuator rod will get it unstuck.

 

[Hollndr]

Grounded the temp switch wire when removed from switch and guage jumped so the guage is good and the temp sensor is good.

Im back to possible brake issues though. I can see my brake lights on when no brakes applied. It did this when my brakes had locked up on me. Truck still drives however (moves around the yard without a dragging feeling). Both parking brake and dummy light work. Not sure if this is cause of my alarm or not. Look at it more this weekend.

 

[The HUlk]

Grounded the temp switch wire when removed from switch and guage jumped so the guage is good and the temp sensor is good

Regarding the alarm, there are two temp sensors.

One sensor is for the temp guage that you grounded causing it to jump. This sensor has nothing to do with the alarm.

The other temp sensor, or grounded wire 36/PIN D if the sensor has been eliminated, is what generates the temp alarm if open circuit. Try grounding PIN D which is wire 36 and if the alarm goes away then you will know it is the temp alarm.

 

[Hollndr]

Grounded "D" wire and buzzer went away... looks like that was my alarm issue. Thanks for putting up with my stupidity HULK. Appreciate your willingness to keep at it with me.

 

[The HUlk]

Grounded "D" wire and buzzer went away... looks like that was my alarm issue. Thanks for putting up with my stupidity HULK. Appreciate your willingness to keep at it with me.

No problem, I knew you could get through it...hoping you or someone else may help me when I need it.

Wire 35 and the sensor are up on top of the motor I believe. Perhaps someone who has actually laid eyes/hands on it can confirm. I would guess they deleted this alarm because it was prone to failure and it didn't prove to prevent much harm.

 

[Hollndr]

Didnt see any sensors that jumped out at me but will take a better look here this afternoon.

On the brake issue, my brake lights will come on once pressure is at 70psi. Dummy light and alarm go out at 60psi as should. When i blow down the air system i can hear all brake cylinders backing off. Brake lights dont go off until all pressure is out of the system. I did hear what sounded like some air bleeding off around the brake pedal in the cab after guage showed 0psi. It wasnt until the air sound went away that the brake lights went off. They didnt come back on until pressure was back to 70psi. Could it be an issue with the brake pedal assembly? Anyone had similar issues?

Take er easy

 

[The HUlk]

Didnt see any sensors that jumped out at me but will take a better look here this afternoon.

On the brake issue, my brake lights will come on once pressure is at 70psi. Dummy light and alarm go out at 60psi as should. When i blow down the air system i can hear all brake cylinders backing off. Brake lights dont go off until all pressure is out of the system. I did hear what sounded like some air bleeding off around the brake pedal in the cab after guage showed 0psi. It wasnt until the air sound went away that the brake lights went off. They didnt come back on until pressure was back to 70psi. Could it be an issue with the brake pedal assembly? Anyone had similar issues?

Take er easy

I don't have much brake system experience but will try to get the ball rolling...

When the brake pedal is pressed air pressure in the service brake system closes the contacts in the stoplight switch which lights up the brake lights. It sounds like at 70PSI something other than pressing the pedal is causing the service brake system to become pressurized and the pressure is held until the system is aired down. Are the brakes applied before 70PSI? Are they applied after 70PSI?

 

[Hollndr]

Brake lights are off before 70psi. Once reach 70psi they stay on until system is completely emptied.

Im not sure if its a switch bad or a small amount of pressure applied to brakes because the truck isnt locked up, i can still drive and move around yard without it feeling like the brakes are applied. However when i did have the brakes lock up on me months ago, the brake lights were on without applying foot brake...

 

[The HUlk]

Brake lights are off before 70psi. Once reach 70psi they stay on until system is completely emptied.

Im not sure if its a switch bad or a small amount of pressure applied to brakes because the truck isnt locked up, i can still drive and move around yard without it feeling like the brakes are applied. However when i did have the brakes lock up on me months ago, the brake lights were on without applying foot brake...

When the brake predal is pressed air pressure causes the brake light switch contacts to close and supply ground to the brake light circuit. I would guess an internal spring opens the contacts when the pressure diminishes. The brake light switch has two wires and is inside the drivers side frame rail between the transfer case and rear corner of the fuel tank.

 

[Csm Davis]

Okay Hollndr stop. Backup and start over, did you have a vent on your passenger side front gladhand? Next if you haven't, change the big filter on the air drier and remove the valve and heater and cartridge from the bottom of the drier and clean it all and if you want to and it looks like it needs it replace the cartridge but most don't need replacing just yet. Now go under the truck and remove all of the check valves from all of the tanks and clean and clear them and the air lines they attached to, while you are looking check your pressure sensors I have seen many failures of these. This is what I would suggest that everyone that has an air issue or brakes problem start with, this will be the cause of most problems and you will be sure you air system is clean and functional.
I didn't sleep at the Holiday Inn Express last night but I have moved thousands of these trucks.

 

[Hollndr]

Recap of things ive done thus far on brake issue. Replaced my front non vented gladhand with a vented a few months ago. (Still not sure why this is needed when the truck operated without issue for a year with a non vented, but did anyways). Pressure sensors checked out good. Removed some of the lines and fittings at tank to check for obstructions, all i checked were good.

Will hopefully get some time to do some more looking this weekend before snow hits.

 

[Hollndr]

Brake light being on issue has been solved.

Id like to thank everyone for all your input and help, i greatly appreciate it.

Now onto the resolution... let me be the first to call myself a dummy. Long story short, my 2 and 3yr olds were playing in the cab while i was troubleshooting the overtemp alarm i was getting and one of them must have "bumped" the trailer lever on the column down a bit... not enough for me to notice but enough to apply brake lights. So... something else to consider when having brake issues if brake lights are on and truck still moves.

Thanks again everyone.

Take er easy

 

[Csm Davis]

Recap of things ive done thus far on brake issue. Replaced my front non vented gladhand with a vented a few months ago. (Still not sure why this is needed when the truck operated without issue for a year with a non vented, but did anyways). Pressure sensors checked out good. Removed some of the lines and fittings at tank to check for obstructions, all i checked were good.

Will hopefully get some time to do some more looking this weekend before snow hits.

The vented gladhand is a slow build up kinda thing it builds a little pressure each time you let of the brakes and the cover by itself won't hold a lot of pressure if it isn't real tight, so it could have been missed as a problem builds pretty quickly when Beatle Bailey puts a valve in the line and shuts it off.
The main line going in between the wet tank and the air drier is the main one that gets a lot of rust and crap in the check valve and line, but have seen it happen elsewhere also. Also check and make sure that the two valves on the wet tank are open completely.