Duramax Power Project
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Disc Brake Upgrade - Part II

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By Jim Bigley

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A rear disc conversion generally offers better braking ability - both when dry and especially when wet, weighs less than most drum brakes, sheds heat faster - more fade resistant, and discs just look way cooler... Our rear disc brake installation is now complete. Read on to find out how they look and stop.

We converted to rear disc brakes primarily because we had upgraded to a stronger rear axle, were using a larger tire/wheel combination, and we needed better rear braking than was provided by the original rear drums. The sum of the new rear disc brake components provided no weight savings when compared to the small 10" drums used on the original 10-bolt rear axle, but in addition to the coolness factor, the rear braking ability has definitely improved - more about that later. Part I of this disc brake conversion series was presented last time, and which complements this update.

There are a number of companies that offer disc brake conversion kits for 1500/2500/3500 series GM pickup trucks and SUV's currently equipped with rear drums (see list at end of article). A DIY project like this, when using all new GM parts + fabrication, will cost about the same as a commercial kit. I chose to use all new OEM parts for ease of maintenance and when buying future replacement parts. Plus, a functional and effective emergency brake was necessary because this truck will be trailered occasionally. The e-brakes included with most aftermarket disc brake kits I've seen don't inspire a lot of confidence.

Fabricating the offset brackets was the most time consuming portion of the project. I had a local fab shop build these for me, using my drawing. The ½" steel end plates were cut from sheet using a water jet CNC, and a section of steel pipe was located that fit the OD of the axle tubing.

Once back from the fab shop, I spent some time drilling and tapping the sixteen holes for the 12mm bolts, then squirted on a little black paint.

It should be noted that the dimensions listed on the drawing fit the 1997 model-year 9-½" 14-bolt semi-float axle used in our buildup. If you are installing similar disc brake components on the smaller GM 10-bolt rear axle, you may need to adjust the thickness of the offset brackets to compensate for a different axle flange location. Flange offset bracket thickness is correct when the rotors are centered in the assembled disc brake pad brackets.

The newly fabricated offset bracket has been bolted onto the axle flange using four 12mm bolts, each torqued to 100 ft/lbs using thread locker. Correctly clocking the square ends was important to transfer the correct orientation to the disc brake backing plate.

I drilled out the threaded holes in the original axle flange (using a ½" bit) to allow the 12mm bolts to pass through and then thread into the new brackets.

The disc brake backing plate has been bolted on using four 12mm bolts, each torqued to 100 ft/lbs using thread locker. The calipers and disc pad brackets are bolted onto the forward-facing side of the backing plate, 9 o'clock on the driver's side and 3 o'clock on the passenger side (passenger side shown).

The axle shafts need to be removed when installing the offset brackets and disc brake backing plates. It's a fairly simple job to remove the differential carrier pin, slide the axle toward the center, then remove the axle-retaining C-clip. The axle will then slide out. Be a little careful not to damage the wheel bearing seal.

The original steel hydraulic brake lines were carefully reshaped by hand to fit the disc brake flex-line mounts. The truck's original e-brake cables can't be modified to work with these GM disc brakes due to the new rear facing e-brake cable anchor (1) and actuation lever (2).

To match the new disc brakes, you'll need a set of e-brake cables made for a similar wheelbase 1500 series truck (i.e. 2000 model-year, models 03, 06, 53 - new style 4x4). These loop over/under the axle, then sweep around to make the connection on each side.

This driver's side photo shows the e-brake cable sweeping around and anchoring to the shock mount, then running under the axle tube, before sweeping up and through the wire loop bolted to the frame.

On the passenger side, the e-brake attaches to the rear facing side of the brake backing plate, loops away and then is anchored to the axle tube, before sweeping up and over the pumpkin. The cable mount shown in this photo that secures the cable to the axle tube is simply a steel strap, but you can weld on a tab if you have access to the right fabrication equipment.

A metal clamp secures the e-brake cable from one bolt on the differential cover. From there, the cable sweep across and forward through the wire loop on the frame.

The forward ends of both e-brake cables mount to a fabricated bracket that is bolted to the frame. I may tinker with it a bit over the next few months.

A 2" long piece of ½" square steel tubing was used to provide an anchor point for both cables that can rotate slightly to accommodate a slight misadjustment between the right & left e-brake shoes. This is similar to how the factory parts operate.

Articles have appeared in various truck magazines that mentioned a few potential complications when converting from drums to discs. First, some recommend a new brake master cylinder designed for a 4-wheel disc system, to either provide more fluid volume for the larger rear disc brake caliper pistons or to modify hydraulic brake pressure. Secondarily, they mention that the front/rear brake proportioning valve may need to be modified or replaced for some installations. I had installed a new brake master cylinder and hydraulic brake booster when the truck was assembled a year ago, and I did not want to buy replacements. After breaking-in the rear rotors/pads and performing a few hard stops, I feel that (for this particular truck and installation) a new master cylinder is not necessary, nor is replacing/modifying the proportioning valve. The current front/rear brake proportioning appears to be nearly ideal for this lightweight truck.

I lucked out, but some of the factors that contribute to correct brake proportioning include the weight of the vehicle - especially over the rear, condition of the brake master cylinder and hydraulic brake booster, wheel & tire combination and type/composition of the disc brake pads themselves (some are more aggressive than others). I installed the NAPA Premium Disc Brake Pads #AE-7662-M, which are asbestos-free and are supplied with new anti-rattle clips.

An alternative to retrofitting discs to your existing rear axle would be to locate a suitable disc brake equipped rear axle that had been installed in a newer vehicle. If you can locate a replacement disc brake equipped rear axle in good shape and with the correct gearing, your total cost could be somewhere in the same general price range as a rear disc brake conversion kit. I considered going that route, but unfortunately, GM didn't produce a six-lug disc brake equipped rear axle with the 9-½" ring & pinion that I needed (all current GM disc brake equipped 9-½" rear axles now have 8-lugs, and are used on GM's light-duty ¾-ton trucks).

If you have a drum brake equipped heavy-duty ¾-ton or 1-ton truck, you'll find a variety of aftermarket disc brake upgrade kits currently available for your rear axle. In addition to the braking advantage, disc brakes would provide a worthwhile weight savings due to the much larger drums used on the HD vehicles. Would I do it again on a ½-ton? Tough call, unless you want/need the extra braking performance or just want more modern binders on the back. They do look cool!  TDP