Rear brakes release under hard braking
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Joined: Jul 2002
Posts: 2,242
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From: Celina, TX
Watch your speedo- does it register from 1-2 mph up or does it "jump" up to a certain speed like 10 mph?
If the latter, do a proper axle oil change with a cleaning of the sensor in the diff.
On a 20 year old truck I would not try to just pull the sensor from the top, the bolt likes to snap off.
If the sensor has a beard of metal you get no reliable speed signal, and the RWALB would rather have you brake moderately than spinning off the road.
In some rare cases I had a similar phenomenon due to the proportioning valve that opened too much due to the rear axle unloading.
(You wrote that you don't have one, but I think this is curious, it should sit in the driver's side frame rail slightly forward of the axle. There should be a lever connecting to a pushrod that goes to the axle. Maybe someone just wired the lever up or something like that)
If the latter, do a proper axle oil change with a cleaning of the sensor in the diff.
On a 20 year old truck I would not try to just pull the sensor from the top, the bolt likes to snap off.
If the sensor has a beard of metal you get no reliable speed signal, and the RWALB would rather have you brake moderately than spinning off the road.
In some rare cases I had a similar phenomenon due to the proportioning valve that opened too much due to the rear axle unloading.
(You wrote that you don't have one, but I think this is curious, it should sit in the driver's side frame rail slightly forward of the axle. There should be a lever connecting to a pushrod that goes to the axle. Maybe someone just wired the lever up or something like that)
As for the load sensor/proportioning valve, it doesn't exist on my 2001. I checked my FSM and there is no reference to it, just the proportioning valve that is built into the combination valve. I've also been over all the brake lines and there is nothing between the front and rear as a valve.
I was playing with it a bit tonight. When it would "release", I would try to push harder on the brake but it was pretty solid. I could release and reapply, but it wouldn't give much. I tried a few hard hits and it would lock the front wheels briefly and then release them. I'm going to need to find a scanner to check for codes, or better yet see what the ABS computer is actually seeing. Don't want to just throw random parts at it, especially if is the electronics.
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Joined: Nov 2001
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From: Austria Europe
Well, that was my brainfart then- 4WABS does not have the load sensing valve.
On 4WABS I have found that even the slightest bit of air in the lines, or drums that aren't adjusted properly lead to the problems you describe.
I prefer to fill those vehicles with DOT4 SL.6 brake fluid, it has a much lower viscosity and better lubrication characteristics.
You may have to activate the ABS module with a scanner to get a proper brake bleed on this truck, air stuck in the ABS module can drive you mad.
On 4WABS I have found that even the slightest bit of air in the lines, or drums that aren't adjusted properly lead to the problems you describe.
I prefer to fill those vehicles with DOT4 SL.6 brake fluid, it has a much lower viscosity and better lubrication characteristics.
You may have to activate the ABS module with a scanner to get a proper brake bleed on this truck, air stuck in the ABS module can drive you mad.
Thread Starter
Registered User
Joined: Jul 2002
Posts: 2,242
Likes: 16
From: Celina, TX
So that’s the difference! I always knew there was no valve, but this is their first time I’ve seen why it’s on some and not others. It’s never mentioned in my FSM, so I just assumed it wasn’t on a 2001.
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