05 Tahoe with JL4 RPO rear brake issue

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Boostaddict91

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Hi all,

Just picked up a 05 tahoe LT with the auto ride and active braking, now the trucks rear calipers were not releasing to the point where it destroyed the pads, when I removed them the friction material fell away in chunks. It turns out whoever just did the brakes within the last year replace all 4 corners with new everything (pads, rotors, calipers, and hoses). It turned out that the hoses were for the non active braking model, replaced calipers pads rotors and hoses with the correct ones but the calipers still arent fully releasing. It has hydroboost but from everything that I've found hydro boost w/ active braking isnt supposed to have the combination/proportioning valve. However mine does and the only thing I keep getting is the service stability message for the precharge pump but no physical abs or t/c light on dash... I'm out of ideas any insight would be greatly appreciated.

Thanks.
 

nonickatall

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I don't know details around the GMT 800 brake system, but from my point of view the Hydroboost system has nothing to do with the brake system itself. Only the brake force amplification does that. It may be that only other hoses are installed, because they have to be more pressure-resistant.

If rear brake calipers do not open sufficiently, in my view there are 4 possible causes.

1. Brake fluid that has not been changed for a long time and contains water and leads to corrosion in the brake caliper piston.
2. Swollen brake hoses or crushed brake lines.
3. Defective regulator valves installed on some cars to increase the braking effect of the rear brakes, when loaded.
4. ABS blocks where valves hang, or are sluggish.

Since you have changed the brake calipers and hoses, I assume, that the car has received fresh brake fluid. So points 1 and 2 fall away with you. I don't know if the GMT 800 has a regulator valve at the rear, otherwise the only thing left with you is that your ABS block valves do not open properly inside and disable to release the pressure to the main zylinder. This can be a mechanical issue or electrical or software problem, but as you don't receive Abs light, i guess it's a mechanical issue.

Excuse my English i'm from Germany



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swathdiver

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There may be a problem with the ABS pump... I see our German friend mentioned that. Bump this post when folks are having their morning coffee and they will know how to disable the ABS so you can further diagnose this and maybe drive it if necessary. A Tech-2 would be handy here and might just take out the guesswork.
 
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Boostaddict91

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I dont have a tech-2 but I do have aftermarket scanner with gm software, fluid is fresh, as this truck less than a year ago had the entire brake system overhauled including the hydrobooster and master cylinder. The only code I randomly get is for the precharge pump but thats hit or miss, after installing this second set of rear calipers with the proper hoses I've definitely got better brake pedal feel and the precharge pump doesnt run everytime I let off the brakes (I think this is why it kept throwing the stability service) I just did about 3ish mile drive when I parked it the rear calipers are now only slightly warm to the touch I can definitely put my hand on them so maybe it was the hoses.
 

nonickatall

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I dont have a tech-2 but I do have aftermarket scanner with gm software, fluid is fresh, as this truck less than a year ago had the entire brake system overhauled including the hydrobooster and master cylinder. The only code I randomly get is for the precharge pump but thats hit or miss, after installing this second set of rear calipers with the proper hoses I've definitely got better brake pedal feel and the precharge pump doesnt run everytime I let off the brakes (I think this is why it kept throwing the stability service) I just did about 3ish mile drive when I parked it the rear calipers are now only slightly warm to the touch I can definitely put my hand on them so maybe it was the hoses.
The preload pump is in your ABS Block, so it seem that you find there the solution to you problem.. :thumbsup:
 

nonickatall

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This truck has two. The precharge is in the engine bay, the actual abs pump is mounted on the frame rail. Only the JL4 trucks came this way, and apparently from my research you cant find new pumps either, just the regular abs pump.
Interesting. What are two pumps for?

From my knowledge, most cars have a ABS Block which is the combination of the ABS control unit, the pump and the magnetic valves. Some cars have a several ABS Unit, where only the controler is inside, and a hydraulic block, where pump and magentic valves are integrated.

The pump normalwise makes that typical stutter of the ABS.

What is the second pump for?
 

nonickatall

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Ok I found out, it's for additional pressure at certain purposes..

I found a good document which explain function...


But I think when the abs not release the pressure from rear callipers, then the magnetic valves, not the pump are the problem.

But probably the problem is fixed, when the brakes after a trip are not hot like he wrote.

Normalwise when you drive in normal circumstance, not only stop and go with fast acceleration and strong brake down, or down the hill, so to speak normal road or highway just brake normal. You wheels should be warm, all equal, not hot. If only one wheel, or the rear wheels are so hot, that you can't touch them with bare hand, there is a problem.
 
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Boostaddict91

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So it's been a few days and now it appears to be sticking again on both back calipers. Not constantly but enough that after 16 miles they are warm to the touch.
 
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Boostaddict91

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Also could the fact that it has 07+ 20" LTZ wheels on it make the abs act screwy?
 
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