96' K1500 Braking HELL

[evilunclegrimace]

Scary, isn't it?


Yup.

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^^^ 11.x inch Duo-Servo brakes. Primary shoe is shorter, softer, thicker, often lighter-colored. Secondary shoe is longer, harder, thinner, generally dark-colored. The purpose of the primary shoe is to cram the secondary shoe into the drum harder than the hydraulic system alone can. This is called "self-energizing". If it's TOO effective, the brakes lock up. If it's MUCH TOO EFFECTIVE, the brakes stay applied when you pull your foot off the brake pedal. Happens more often when the brake return springs get weak, AND the shoes get contaminated with brake fluid or gear lube.

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Leading/Trailing shoe brakes. The leading shoe does almost all the braking going forward. The trailing shoe does almost all the braking in reverse. Guess which shoe tends to wear-out faster. Thus some Leading-Trailing shoes having more material on the front shoe.

This design is also self-energizing in that the main braking shoe (front going forward) is dragged "down" by the rotation of the drum and wedges into place based on the geometry of the anchor points. The less active shoe (rear shoe when going forward) tends to get wedged "upward" by the rotation of the drum, which helps braking power directly, but also pushes on the more-active shoe through the wheel cylinder hydraulics.

Same deal--if the shoes get contaminated, the self-energizing action can make the wheel skid, or the brakes fail to release.

GM, Ford, Chrysler, and AMC went decades using Duo-Servo brakes on their main fleet of vehicles. Why they switched to Leading/Trailing drums on smaller cars, I have no genuine knowledge.

My best guess it that they needed some design that would work for a park brake, but NOT work as well on the back of wrong-wheel-drive vehicles so they didn't overpower the lightly-loaded rear tires leading to a skid. The GM "X" body (Citation, and clones) had problems with the park brake power vs. service brake power--if the shoes were aggressive enough to hold the vehicle when the park brake was set, they would lock the wheels in a panic stop. Folks died.

How the leading/trailing design was approved for a pickup is probably similar--when unloaded, all the weight is up-front like a wrong-wheel-drive vehicle. Overly-aggressive rear brakes would make the back end skid, and that's never good unless you're TRYING to spin the vehicle.

But all that is just a guess, and maybe not even a good guess. I wasn't there when the engineers and cost-accountants did their thing.

One of the reasons that GM went to the leading/trailing brakes on the GMT400 platform was the "new" RWAL brake system. The DuoServo braking system by design causes the brake shoes to cam into the drum effectively increasing the stopping force . This caused issues with the RWAL system preventing the shoes from releasing and reappyling quickly.
The leading /trailing style brake system did not cam into the drum as much as the Duoservo system and was able to pulse the shoes allowing the rear brakes to release and reapply quickly to help prevent the rear of the truck from sliding.
Gm did install the RWAL system on trucks with DuoServo brakes but ran into issues with the rear brakes not being able to cycle or pulse causing the rear brakes to lock up in certian road conditions.

 

[Vic327]

Sounds like the rears are locking up. Might be a good idea to double check the PN on the wheel cylinders. If you raid the parts bin you can find 1", 1 1/16" and 1 3/16" under different PN for various models that will bolt in.

From experience I can tell you with 11" drums and 1 3/16" cylinders you will lock up. They would be great if you added a adjustable prop valve. The brakes worked very well until hard braking locks them up even with ABS it will do it in the dry. I switch them out for the 1 1/16" paired with 2500 8600 lbs front calipers, a 1 1/8" master cylinder from the 70s and braided brake lines. Brake are good now but not like modern cars.

 

[Erik the Awful]

Skip that kit and fix your brakes instead. The only thing that kit does is empty your wallet. We've even had a member who sells those kind of rear disc conversion kits say they're no improvement over the stock drums.

Disc brakes are not an upgrade in performance, they're an upgrade in maintenance. A correctly adjusted drum brake has every bit as much braking power as a disc brake. The problem with drums is that they fall out of adjustment. If you maintain your truck and adjust the rear drums whenever you have the rear wheels off, you won't have that problem.

If you want to swap to discs, you'll want to replace the whole brake system, front to rear, with significantly better parts all around instead of just scabbing on discs and rotors from a wrong application.