Air Brake Info and Troubleshooting

Reworked LMTV

Well-known member
Steel Soldiers Supporter

The brakes on these trucks are wedge brakes. One of the guys at the local Truck Pro commented that wedge brakes are obsolete now with other self-adjusting brake systems. He asked if the truck was really old and was surprised when I told him mine was a 93 model. If you are completely new to air brake systems like I was, I suggest you do some reading about how the brake chambers work, it will help quite a bit when you start troubleshooting them.
Wait, wasn't replacing this a bit on the DANGEROUS side?
Here is a link to a Haldex catalog that lists wedge brake chambers: Actuators Catalog 2-2012.pdf

The rear chambers are a trade size 9/12, which means the inside (service) diaphragm is a trade size 9, and the outside (parking) diaphragm is a trade size 12. On these the parking brake portion is removable, although I would probably not recommend it. Due to safety concerns, the parking brake section on most new style chambers is crimped closed so you can't open it. There is info online about the dangers of the parking brake springs if you are curious. The service brake side of the chambers can be accessed, just remember to cage the brakes before you open it.

On my truck, I had and air leak in the rear when applying the service brakes (brake pedal). I was able to trace the air to the vent line that runs up to the frame rail and vents there. This pointed to a leak in the service brake diaphragm which was allowing air into the vent line. You can pick up a size 9 seal for under $10. The only caveat is that with the factory seal, the plunger piece is glued to the seal. Aftermarket seals are not made this way. So, for the cost I decided to just attach the plunger piece back to the new seal. I removed the old seal, peeled it off the plunger, scraped the plunger, wire brushed it, cleaned with alcohol, and applied contact cement. So far so good on the repair. I think there is probably a better adhesive option, but not that I could get locally to fix mine in a hurry.

In this pic, you can see one of the factory seals on the left with the plunger attached. The plastic bushing shown has been removed from the one on the right while I was repairing it. On the right is the aftermarket seal with the plunger piece glued back to it. The factory seal was shaped so that the center was raised, the aftermarket one is not, but works just fine as far as I can tell.
View attachment 562126
Was'nt replacing these a bit dangerous?


Steel Soldiers Supporter
Stoneville, NC
I would be will to bet that 99% of the death was someone did not cage the can before removing the clamps. 1% got careless with the cap when caged.
I would probably just replace the piggyback assembly if the emergency brake side went bad. You could have an accident even doing the service side, but the emergency park spring is completely contained in the rear piggyback section so it's safer than trying to replace the diaphragm on the emergency side.


Well-known member
Steel Soldiers Supporter
Supporting Vendor
Interlachen Fl.
I did a right up about it on the 939 series and I just did my trailer in 20 minutes both sides. The problem with the cans with the safety tabs is it has to go side ways to come off or go on. I was about ready after I got the cap off to take a cut off wheel and cut the safety tabs when I did my 939. I just fought with it and got it right the first time but if I ever do it again I will cut off the safety tabs and install the cap straight on.
Don't do this!!


New member
Las Vegas NV
Location of air gauge transmitters location am a M1078 A1 .

Could anyone point out the location of the air gauge transmitters on a M1078A1 with ABS brakes . On an A0 it's on the left hand side opposite the brake pedal but on an A1 that is a location for part of the ABS system .
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