(Solved) M923A2 Right rear brakes grabbing

[kendelrio]

I drove my 5 ton to pick up my spare and on the way my brakes on the right rear axles kept "grabbing" to the point it would smoke my brakes. (I was only going three miles from the house and the shop is the first turn around).

Coming home, it did it a couple times in the first mile then stopped. Coming up to my driveway, I figured I would downshift the transmission to slow down and when my RPMs jumped from the downshift, the rear of the truck literally slid to the left from the brakes grabbing. Both tires on the rear axles are warmer than usual to the touch, which tells me the brakes are hot as hell. She is parked until I figure this out (which because I leave Sunday will have to be when I get back in three weeks.)

Can anyone point me in the direction to start troubleshooting/repairing this issue?

 

[Elk1111]

I would replace the diaphragms. Mine was doing the same thing on my rear axle and it ended up being the diagram. Might as well do them all at once.

 

[kendelrio]

I would replace the diaphragms. Mine was doing the same thing on my rear axle and it ended up being the diagram. Might as well do them all at once.

Thanks for the heads up. I hate to ask, but with 30 TMs to go through, you wouldn't happen to have a reference TM number and page, would you?

 

[98G]

As Elk said, it may be the emergency side diaphragm leaking, allowing the springbrakes to grab.

One way to test this is to air the truck up, shut it off and listen for leaks and watch your air gauge.

If you decide to replace your diaphragms, also as Elk said, may as well replace all of them, emergency and service. They're something like $15 per set.

As Elk didn't say, and as can't be emphasized enough, taking that brake can apart is pretty simple. But there's a spring in there that wants to kill you. It holds more than enough energy to literally take your head off and will do so at the slightest provocation. If you've never disassembled these before, you may want to invite someone over who has experience...at least for the first one.

Also, strikes me as odd that it's both sides. Makes me suspect that it might be something other than the diaphragms.

 

[kendelrio]

As Elk said, it may be the emergency side diaphragm leaking, allowing the springbrakes to grab.

One way to test this is to air the truck up, shut it off and listen for leaks and watch your air gauge.

If you decide to replace your diaphragms, also as Elk said, may as well replace all of them, emergency and service. They're something like $15 per set.

As Elk didn't say, and as can't be emphasized enough, taking that brake can apart is pretty simple. But there's a spring in there that wants to kill you. It holds more than enough energy to literally take your head off and will do so at the slightest provocation. If you've never disassembled these before, you may want to invite someone over who has experience...at least for the first one.

Also, strikes me as odd that it's both sides. Makes me suspect that it might be something other than the diaphragms.

I watched a YouTube video where the guy said taking it apart would help you meet Jesus.

Before I tear into that, I'm gonna look and see if I can find the commonality between the brakes to see what could cause them both to grab.

Repair by replacement gets expensive pretty quickly.

When I get in and have more time I'll troubleshoot it. Air it up, chock the wheels, shut down without the parking brake on and listen for a leak, then have someone press the brake pedal so I can see which side it's on if there's a leak.

 

[TheQuaker]

[mention]kendelrio [/mention]

Check the vent on the front passenger dummy glad hand. That is a vented cover.

If that vent gets plugged by a mud wasp, debris, etc. the system cannot exhaust properly and will lock up your rears as you describe. Happened to me once and now I always check for that on my pre-trip inspection.


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[kendelrio]

[mention]kendelrio [/mention]

Check the vent on the front passenger dummy glad hand. That is a vented cover.

If that vent gets plugged by a mud wasp, debris, etc. the system cannot exhaust properly and will lock up your rears as you describe. Happened to me once and now I always check for that on my pre-trip inspection.


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By "check" I'm assuming you mean see if air flows through it? I just blew through it and it didn't seem clogged.

 

[TheQuaker]

Do you have just a dummy with a hole or does the dummy have the little covered vent screwed into it? If so, unscrew the little vent, put the dummy back on and take her for a spin and see.

My issue was that tiny valve got a hunk of crud in it and was plugged. Hit it with some brake cleaner to clear it and it worked perfect afterwards.

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[98G]

I watched a YouTube video where the guy said taking it apart would help you meet Jesus.

Before I tear into that, I'm gonna look and see if I can find the commonality between the brakes to see what could cause them both to grab.

Repair by replacement gets expensive pretty quickly.

When I get in and have more time I'll troubleshoot it. Air it up, chock the wheels, shut down without the parking brake on and listen for a leak, then have someone press the brake pedal so I can see which side it's on if there's a leak.

Note that the service side will only leak when pedal is depressed, and the emergency side would leak even without pedal being depressed.

My wrecker has a leak in the service side

 

[kendelrio]

Do you have just a dummy with a hole or does the dummy have the little covered vent screwed into it? If so, unscrew the little vent, put the dummy back on and take her for a spin and see.

My issue was that tiny valve got a hunk of crud in it and was plugged. Hit it with some brake cleaner to clear it and it worked perfect afterwards.

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It has a little spring covered cap with a hole in it. Gimme a few and I'll post pics.

 

[TheQuaker]

It has a little spring covered cap with a hole in it. Gimme a few and I'll post pics.

Just unscrew that valve/cap or remove the dummy coupling entirely from the truck temporarily so it doesn't swing around, take her for a spin and see if that fixes the problem. If so, either ditch the valve/cap, see if you can clean it or just get a shiny new one. If you really want a new one to keep her "original" let me know and I'll send you a factory-new one. I have a couple of spares from an annual maintenance kit I bought on auction awhile back.

If this doesn't fix the problem then follow up on what the others have suggested...but your problem sure sounds like what I ran into...

 

[kendelrio]

Just unscrew that valve/cap or remove the dummy coupling entirely from the truck temporarily so it doesn't swing around, take her for a spin and see if that fixes the problem. If so, either ditch the valve/cap, see if you can clean it or just get a shiny new one. If you really want a new one to keep her "original" let me know and I'll send you a factory-new one. I have a couple of spares from an annual maintenance kit I bought on auction awhile back.

If this doesn't fix the problem then follow up on what the others have suggested...but your problem sure sounds like what I ran into...

Thank you so much! I certainly will!

 

[simp5782]

When you do your diaphrams go to home depot and get some longer bolts that have full thread so that the clamps pull together easier over having to use channel locks to try to get a few threads to show

 

[GopherHill]

This might be a good time to check brake adjustments. If you adjust the anchor pins, be sure the punch marks on the pins face outboard, away from each other. Read the TM carefully.

 

[Elk1111]

Use grade 8 bolts if you are replacing them as Simp said. Anything else will bend and make you nervous. The diaphragms are not bad to replace but they will make you a bit nervous. I wore a full motorcycle helmet and made sure not to point it directly at myself. Get yourself some new caging bolts before doing this also. Napa carries them.

 

[kendelrio]

Thank you so much! I certainly will!

As much as I was hoping this was the issue, it is not to be. Took it for a short spin and they continued to lock up.

Now I have to figure out the commonality between the two rear right axles before I start throwing parts at it.

It seems like it's not so much grabbing as dragging.

Hmm. Off to the TMs...

I appreciate everyone's input, bit again, before I go tearing into the truck, I'm going to try to find the root of the problem before I throw parts at it.

 

[simp5782]

Nothing is shared between right side together.

The T that supplies air pressure to the spring brake cans per axle sends pressure equal to both sides.

If one side has enough air to release then so does the other.

Auto adjusters are probably just sticking and with all the moisture this time of year the rust in the shoes is an over adjust and it can't back itself off. They only auto adjust up not off. So the first time you hit the brakes it adjusted up too much

Another issue could be that 2 brakes aren't working and 2 are stuck cause the spring brake yank isn't getting the full air system pressure. This happens the dual check valve sticking by the drivers step on the frame.

 

[kendelrio]

Nothing is shared between right side together.

The T that supplies air pressure to the spring brake cans per axle sends pressure equal to both sides.

If one side has enough air to release then so does the other.

Auto adjusters are probably just sticking and with all the moisture this time of year the rust in the shoes is an over adjust and it can't back itself off. They only auto adjust up not off. So the first time you hit the brakes it adjusted up too much

Another issue could be that 2 brakes aren't working and 2 are stuck cause the spring brake yank isn't getting the full air system pressure. This happens the dual check valve sticking by the drivers step on the frame.

I just filmed the "hold" test and am uploading it to YouTube. I'll post it in a second.

What I did was pressure up the system, chock the wheels and have my boy press and hold the brakes. It's leaking a LOT of air back there. It seems to be on both sides. I'll post it after it uploads.

 

[kendelrio]

Here's the "hold" test...

 

[simp5782]

Diaphrams

FYI you can chock the wheels.

Hook an air line from the front emergency gladhand to the front service gladhand and it will engage the brakes for you to find leaks on your own if you have no soldier b