M915A1 Brakes Locked Up

[flametamer]

We(vol. fire dept.) have a new to us M915. It is currently on the side of the road with the truck brakes locked up. The M967 trailer we are pulling seems to be fine. The truck parking brake and trailer supply valves seem to be working fine. They stay in/out, & act normally. The foot brake petal, however acts funny. It feels different and when released, some air exists the valve through a vent or whatever at the top of the valve that exits throught the firewall. I tried caging the front drive brakes, but the truck still will not move.

Does anyone have any idea of what could be wrong, or have any troubleshooting tips?

Thanks
Darrin

 

[wreckerman893]

Cage all of the brakes and (very carefully) see if she will move.

If it won't the S cams have prob malfunctioned.

I'm not a mechanic but it's not a hard fix for someone versed in air brake systems.

 

[R Racing]

Id be willing to bet that 1 of your S cam shafts in the rear are frozen or you have a broken can spring . It happens when the truck sits and doesn't get enough greese or used much. they gaul up and lock on. its pretty easy to trouble shoot.First Chock the wheels, Put truck in nutral.while some one actuates the brake pedal . watch for the arms to actuate . they should move about 20 degrees easy. if you see 1 that hardly moves , that would be your sticky 1.

 

[Floridianson]

From what I read in your case could the foot brake air valve be going out ( sticking). Could try and lube that valve or R&R.

 

[M813A1]

Try and see if you have air leaking from a front right glad hand ?? If it is then one of your quick release valves is bad .. Also chock the truck release the brakes and try and loosen up the slack adjuster if the brake rod does not move then your S cam has probablly flipped you maay try and slugging the brake drum with a hammer and they might release if not you will need to replace the S cam !! It might be possible if you can back off the S cam as far as possible and slug the brake drum with a hammer it might release and then bring it back up and you might be able to ge a little brake back so you can move the truck.If not then you will need a Wrecker to move the truck !!

 

[flametamer]

Try and see if you have air leaking from a front right glad hand ?? If it is then one of your quick release valves is bad

Yes it is. Quick release valves where? When I take the brakes off, the brake lights come on. When I pull the valves out, the lights go off. I tried to unhook the trailer to see if the truck would pull out from under it. It wouldn't move. And then the rearend yoke let go.
The front brake pots are releasing, but the brake pads are not. I plulled the inner shields off to be able to look at the pads and drums. The brakes front and rear do not release.

Dt

 

[NDT]

I think we are all somewhat confused. What do you mean the rearend yoke "let go"? Tthe front axle does not have spring brakes that would need to be caged to move a disabled truck. Also, the axles have shoes and not pads. Check out the vehicle's TMs on the JATonka web site to aid in your diagnostics. M813A1 nailed it on the glad hand leaking air, that will cause all the brakes to be applied any time the truck has air pressure.

 

[Irv]

Brakes may be worn out, too

If the brakes are worn down and the drum has been ground so it is larger, it is possible for the S-cam to go too far and lock. You might try this to see if that is what's happening: Push in the tractor air so the tractor releases the brake cans. Check visually to see if the brake rods sticking out of the cans are extended out way far. If so, there is insufficient return spring force to overcome the cammed-over S-cams. Chock the wheels first, and be all aired up, of course.

Irv

 

[flametamer]

I think we are all somewhat confused. What do you mean the rearend yoke "let go"? Tthe front axle does not have spring brakes that would need to be caged to move a disabled truck.

The yoke broke. I meant the front drive axle. Sorry.


Dt

 

[M813A1]

I know because I had the exact problem with a M915 A1 recovering trailers from Red Rock Army Depot in Hooks Tx. On my way home I stopped and when I pushed in the trailer and truck knobs the brakes would not release so a friend found the air leaking from the right front glad hand so he put his thumb over the opening and let the pressure build up and it released the brakes and I was able to move out again !! After getting back to OKC the truck was put into the shop and te mechanic found a bad quick release valve was bad !!

 

[cornrichard]

There is a quick release valve on the frame that does the parking brake and the service brakes. It is problematic from time to time. You might try putting two regular quick release valves in its place. This will only help if all 4 rear brakes on the tractor are stuck. If the parking brakes on the front tandem are the only ones locked you have a different problem.

 

[Autocar]

My 915A1 has parking brakes on the front drive axle only.

 

[cornrichard]

That was the point I was trying to make. If it is just the front tandems, it is a parking brake problem. If it is all four, then something is up on the service brake side. I should have been more clear.

 

[flametamer]

I finally got it to move. The park brake is working backwards. I had to leave the truck park brake on, and supply the trailer with air to unlock it to drive it. When you take the parking brake off the truck(by pushing the valve in) it sets the brakes and the brake lights come on.

Dt

 

[R Racing]

thats a weird find ! well at least you know whats going on now.

 

[73m819]

do you have the lines crossed at the trailer, try switching

 

[flametamer]

The trailer brakes work fine. I tried swapping them to see but no luck. I ended up running a line from one of the tanks straight to the trailer to get it rolling and eased the truck down the road in 2nd to limp it back to the fire station.

Dt

 

[flametamer]

There is a quick release valve on the frame that does the parking brake and the service brakes. It is problematic from time to time. You might try putting two regular quick release valves in its place. This will only help if all 4 rear brakes on the tractor are stuck. If the parking brakes on the front tandem are the only ones locked you have a different problem.

I forgot to update. The valve on the frame was the problem. The shop also found out that the rear axles brakes were inop. It took them a while to find a replacement valve that would work but they finally did.

Dt