Ad Radar
Car Craft
Click here to find out more!

GM Rear Drum to Disc Brake Swap

Stock Rear Brakes Giving You Fits? Stock rear brakes giving you fits? We show you how to get big brakes for 10- and 12- bolt axles.

By , Photography by

Foot to the floor for 100 feet when you brake for that bus full of nuns? Upgrade when we show you that putting disc brakes on the rear of a GM car is actually very easy. Watch as we swap out the drums on our '86 Caprice and swap in a set of discs from a '94 Caprice police car. The good news is that these tricks will work for any GM C-clip-style 10- or 12-bolt axle on nearly every GM car.

Reality Check
You're probably thinking, "That was too easy," and you're right. Yes, mechanically, the job is simple. You can swap drums for discs in a couple of hours as long as you do your research ahead of time. However, a lot of things can go wrong if you don't.

The axlehousing dimensions are the same on all '77-'96 Chevrolet B-bodies (Chevrolet Caprice and impala) are nearly identical under their sheetmetal, and virtually all parts are interchangeable among those model years. We can swap the whole thing into our '86 Caprice, and in doing so, we'll be upgrading from our puny 7.5-inch 10-bolt to a much stronger 8.5- inch unit with limited slip. The rear discs are just a bonus.

Truthfully, we planned to swap axle assemblies all along. The point of the exercise at the beginning of the article was to demonstrate how quickly you could get mired down in the multitude of variables involved in a rear disc brake swap.

Have you ever wondered why there are so many aftermarket companies offering brake upgrade kits? Or why most car builders or restorers, guys who have no fear of building engines and transmissions, will run aftermarket braking systems on their cars? Well, neither did we until we started researching for this article. We quickly discovered that trying to cobble together a brake system involves a lot of trial-fitting until you get the combination right. Fine if you've got the time and funds.

Place Your Order
When swapping brakes, the after-market really does come to the rescue. The brake companies have done the legwork, and they know what parts work best with each other and what will fit your car. We talked to the guys at Baer Brakes, Stainless Steel Brake Co. (SSBC), and KORE3 to get their advice.

They were all in agreement. You can see substantial gains in braking performance when switching to a four-wheel disc system. But there are a few key things to pay attention to. First, everyone stressed how important it is to design the rear system based on what brakes are on the front wheels.

"It's got to match the front system," says Bill Cummings of SSBC. Baer's Ben O'Connor agrees, "it's better to have too small a caliper on the rear than too big. With bigger calipers, you get too much clamping force, too much pedal travel, and the system is very hard to adjust." Because braking physics shift most of the car's weight to the front wheels, the rear brakes don't need to do as much work as the front brakes do. if you have too much clamping force on the rear brakes, the rear wheels will lock and the car can easily start to rotate. in other words, the rear caliper piston(s) must be smaller than the front or you'll be doing 180s every time you touch the brake pedal.

All three companies we spoke to offer basic rear disc brake kits that work in conjunction with stock or modified front systems. O'Connor says Baer's rear kits are modestly sized so as not to overwhelm a stock front system, but they can be easily upgraded if the owner decides to go big on the front brakes. SSBC and kORe3 have similar systems, and some can fit inside your stock wheels.

The components used are a second concern. SSBC's kits utilize a caliper similar to what GM offered on the Cadillac Seville and Firebird WS6. These calipers feature an integral, mechanical parking brake mechanism. engaging the parking brake pulls a lever on the back of the caliper that pushes on the caliper piston, clamping the pads to the rotor. Baer's kit uses components similar to C4 Corvettes and '92-'97 F-cars. They consist of PBR calipers with an improved manual parking brake design that engages a pair of pins, which push in the inboard pad, clamping the rotor. kORe3's systems offer a setup using C5 and C6 Corvette PBR calipers and a drum-inhat-style parking brake. The parking brake is actually a separate drum brake inside the hub of the rotor. kORe3's Tobin knighton prefers this system, saying the drum-in-hat system is less complicated and easier to maintain. The guys at Baer and SSBC acknowledge that the mechanical caliper parking brake arrangement is more complicated, but the system is less expensive to engineer because the parts have been around since the late '70s. Originally, the factory system experienced failure due to corrosion and lack of maintenance. The calipers would come out of adjustment or seize up, rendering them ineffective. Baer and SSBC have had lots of time to work most of the bugs out of the mechanism though, and they say their caliper designs are superior to the original GM pieces. Both Baer and SSBC offer higher-end kits that come with drumin- hat-style parking brakes, too.

Which system is best for you?
Obviously that depends on your budget and driving style. We can say with confidence that they all make a good product. Below are kORe3's, Baer's, and SSBC's best-selling kits. Note that these are not their least expensive kits. Go to each company's Web site for options and specific pricing information.

Junkyard Systems
As mentioned before, enterprising sorts may wish to build a rear disc system using junkyard parts. The old way of swapping out your drums to discs was to acquire the setup on an '80-'86 Cadillac Seville, a '79-'85 Cadillac eldorado, or a '79-'81 Firebird with the WS6 handling package. These cars were equipped with 111.2-inch rear brake rotors. The Caddy used a 5x5 bolt pattern, while the Firebirds were the conventional 5x43.4. So the hot ticket was to grab the calipers and brackets off a Caddy and the rotors from a Firebird. Those parts are still available. A recent trip to a local pickyour-part turned up at least eight complete sets of Seville brakes. The WS6 rotors are much harder to find in the 'yard.

Newer cars offer better systems, and late-model F-car swaps are becoming popular, especially the brakes from lS1 cars. These systems include PBR calipers on 12-inch rotors with a drum-inhat- style parking brake. The rotors are drilled with the common 5x43.4 bolt pattern, making this a popular swap for older musclecars. Good luck finding them in the junkyards, though. The parts are too desirable, so eBay will be your best bet for finding those setups. Also, these brakes will not fit behind most 15-inch wheels, so plan on bigger wheels if you're contemplating this swap.

Surprisingly, the biggest drawback of a junkyard system is cost. Brake parts operate in a harsh, dirty environment under extreme temperature ranges. Therefore, the parts can be totally clapped out by the time you pull them from the 'yard. While we embrace the practice of hosing off junkyard parts and putting them to use, we cannot advocate cutting corners with your braking system. So by the time you've overhauled the calipers, sliders, mounting brackets, dust boots, and parking brake cables, you may well have spent as much or more than it would have cost to buy an aftermarket kit.

What's A PBR Caliper?
PBR is an Australian company specializing in automotive and light-truck braking systems. The systems utilize a floating caliper supported by a caliper bracket that bolts to the spindle or axle flange. Their simple design and adaptability to accommodate a variety of rotor diameters by using taller caliper brackets make them a popular choice for both the manufacturers and aftermarket alike. PBR brakes were standard equipment of many performance cars including the Corvette, Camaro, Firebird, and Mustang SVT Cobra. In addition to calipers, PBR manufactures rotors, parking brake assemblies, and brake lines and hoses.

Measurements You Need To Know
GM used the same mounting flange for most of its 10- and 12-bolt rear axles with C-clip axle retainers. The flange bolts form a trapezoid shape measuring 31.8 inches across the top and 25.8 inches across the bottom. There are a few variations to watch for: Some station wagons, for example, may have a bigger flange. To be safe, take a measurement of your car and the potential donor's.

In addition, be sure to measure your wheel-stud bolt pattern, the pattern of the donor rotors, and the pilot hole in the center of the rotor because some axles have a bigger pilot than others.

Finally, be aware of possible interference of components. Cars with staggered shocks may need to mount the calipers in different clocking positions to avoid contacting the shocks during suspension travel.

  • «
  • |
  • 1
  • |
  • 2
  • |
  • 3
  • |
  • View Full Article
Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!
0 comments
Car Craft