Possibly retro-verting M923 from ABS to NON-ABS

Anvilface

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Wilmington Nc
I know that this might seem off, and I'll probably get an earful from some. (Getting chewed out is ok, I've been chewed out before).
This might be a bit long, so I apologize for that. I just want to give all info and variables in case anyone is inclined to help.

The problem: We had a problem with others in the past draining wet tank, as a result we had several components go bad. The initial issue was that everything worked fine (except ABS light on, and has been for years) until one day while driving, use the brake, all air completely purged after two applications of brake pedal. Then the truck would not build air at all. Connect shop air and still not building.

1. Treadle valve leaking internally (replaced). Truck still won't build air on it's own or with shop air.
2. Qr2 valve checked and determined to be bad (plastic ball ground to little bits) replaced.
3. Leaking diaphram on front service chamber. Broke down and changed all 10 service diaphrams
4. Checked the transfer case shifter and related valve. Seemed to be pulled out just a hair, insured that it was fully seated.

At this point, still would not build air on it's own. The unloader valve and compressor seem to be good based on TM manual tests. Have not inserted a pressure test gage yet, but still may for future testing. However, it will now hold some air when shop air is applied. Secondary builds and holds fine, Primary will try to build but not hold.

With this development, I found air coming from one of the rear relay valves that also is connected to the ABS control crap. I was instructed by my boss to just order and replace that valve since everything else seems to have gone to the dogs along the line. I don't think it will hurt anything to replace it, but I'm not totally convinced that this will stop the problem.

So...after War and Peace being written above, the question I have is "Could this be related to the ABS controls, or do I possibly have just another valve...leak...venting issue further along?"

You guys have always been great with help and knowledge, so any help at all would be fantastic. I'm a former 3521 with the Marine Corps and worked on these pretty extensively, but I have never had one have a complete air system failure like this.
 

simp5782

Feo, Fuerte y Formal
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You just need two relay valves R8 style part number 288781. Swap them in place of both of yours to become non abs valves


Remove the air line from the compressor and insert your shop air there. See if your tanks will build air. The discharge tubes are bad about getting stopped up with crud and cause a slow building of air.

After doing this. If the tanks show air building close the shut off valves on the wet tank for the secondary side . this is on top of the wet tank by the frame and see how your primary holds. We want to isolate one side of the system. There are valves that allow secondary and primary to share.

Primary air is fed to the rear axle brake relay valve first.

Does the primary hold with the parking brake on or off? If it does released report back if it does not hold air with the parking brake on remove the line from right behind the drivers step that goes up under the cab and plug it.

You said you replaced all 10 service diaphragms but did you replace the spring diaphragms? These are fed by the primary circuit with the parking brake off.
 

juanprado

Well-known member
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Abs is your friend.
All air brake components are civy items made by haldex and easily found at napa or HD stores.
After finding problem, I would recommend changing filter on dryer. Decaissant is probably shot.
 

Anvilface

Member
51
7
8
Location
Wilmington Nc
You just need two relay valves R8 style part number 288781. Swap them in place of both of yours to become non abs valves


Remove the air line from the compressor and insert your shop air there. See if your tanks will build air. The discharge tubes are bad about getting stopped up with crud and cause a slow building of air.

After doing this. If the tanks show air building close the shut off valves on the wet tank for the secondary side . this is on top of the wet tank by the frame and see how your primary holds. We want to isolate one side of the system. There are valves that allow secondary and primary to share.

Primary air is fed to the rear axle brake relay valve first.

Does the primary hold with the parking brake on or off? If it does released report back if it does not hold air with the parking brake on remove the line from right behind the drivers step that goes up under the cab and plug it.

You said you replaced all 10 service diaphragms but did you replace the spring diaphragms? These are fed by the primary circuit with the parking brake off.
Thanks Simp,

Secondary does hold air with the parking brake on, the primary leaks out of the check valve on the intermediate axle (the one I was told to replace). I haven't checked whether or not it holds with the parking brake released, but will try that for sure.

As far as the compressor side, there is no doubt that it could be gunked up. That too will be checked.
 

Mullaney

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Charlotte NC
I didn't change the spring side diaphrams, just the service side. I know what those springs can do....and I got skeered..LOL.
.
Those cage bolts are what pull that big monster spring "into the cage". Or maybe the caged position.

Insert the proper cage bolts,
turn the bolts 90 degrees to fit the "pocket" they fit in,
snug the cage bolts with a 3/4 inch wrench until they don't turn any more.
- GOAL is to restrain the springs.

I generally bump the back of the cage bolt just to make sure they are seated after a few turns with the wrench.

When you are caged, the band clamp that you already know about for the service side gets unbolted.

I personally keep my face out of the way.

Spring shouldn't be able to come loose.
Remember that if you keep your soft people parts (face, fingers,etc) as much out of the way as possible - you shouldn't get injured - even if something stooopid happens. When you have the first caged can in your hands, you can see it better and understand the same...

New way of saying it: Treat that spring can like a gun.
You wouldn't look down the barrel of a loaded gun.
 

Anvilface

Member
51
7
8
Location
Wilmington Nc
.
Those cage bolts are what pull that big monster spring "into the cage". Or maybe the caged position.

Insert the proper cage bolts,
turn the bolts 90 degrees to fit the "pocket" they fit in,
snug the cage bolts with a 3/4 inch wrench until they don't turn any more.
- GOAL is to restrain the springs.

I generally bump the back of the cage bolt just to make sure they are seated after a few turns with the wrench.

When you are caged, the band clamp that you already know about for the service side gets unbolted.

I personally keep my face out of the way.

Spring shouldn't be able to come loose.
Remember that if you keep your soft people parts (face, fingers,etc) as much out of the way as possible - you shouldn't get injured - even if something stooopid happens. When you have the first caged can in your hands, you can see it better and understand the same...

New way of saying it: Treat that spring can like a gun.
You wouldn't look down the barrel of a loaded gun.
Yeah, I'm familiar with caging them, and know (in theory) that the spring won't come out. But with the "treating it like a gun" analogy....that's what I'm respectful of. LOL I might not have the prettiest face, but I'd like to keep it where it is.
 

Anvilface

Member
51
7
8
Location
Wilmington Nc
You just need two relay valves R8 style part number 288781. Swap them in place of both of yours to become non abs valves


Remove the air line from the compressor and insert your shop air there. See if your tanks will build air. The discharge tubes are bad about getting stopped up with crud and cause a slow building of air.

After doing this. If the tanks show air building close the shut off valves on the wet tank for the secondary side . this is on top of the wet tank by the frame and see how your primary holds. We want to isolate one side of the system. There are valves that allow secondary and primary to share.

Primary air is fed to the rear axle brake relay valve first.

Does the primary hold with the parking brake on or off? If it does released report back if it does not hold air with the parking brake on remove the line from right behind the drivers step that goes up under the cab and plug it.

You said you replaced all 10 service diaphragms but did you replace the spring diaphragms? These are fed by the primary circuit with the parking brake off.
ok, I drove back out to the shop and checked some GO/ NOGO situations. and here she goes...

at wet tank: Primary closed
Secondary open
will build air in secondary tank and hold pressure pretty well with parking brake both on and released. Also holds if I engage spring brake override.

if I open the primary tank up from the wet tank, air just dumps from the check valve I mentioned earlier, and that happens whether the parking brake is set or not.
 
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