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How To Convert GM 14-bolt from Drum to Disc Brakes

Posted in How To: Suspension Brakes on October 4, 2017
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Jeremy Richmond was facing the same dilemma as many have when making wholesale suspension and drivetrain mods in order to run healthy motors and tall tires on older 4x4s: Stopping power. His well-founded fear was the stock drum brakes on the GM Corporate 14-Bolt he’d slipped under his soon-to-be mud runner and trail truck wouldn’t be up to the task of providing the braking needed to bring 37s to a halt like he wanted.

Drum-style brakes on older 4x4 trucks are adequate for stock applications. But they are woefully poor performers when faced with stopping the rotational mass of taller, heavier tires, be that on the street or off-road. Drum brakes trap the heat generated under braking, which leads to quick fade. They also trap mud and water inside, leading to rapid wear when subjected to mud and water commonly encountered on backroads and OHV trails. Disc brakes don’t have those inherent issues.

The most common direction serious enthusiasts take to remedy those issues on the off-road friendly, full-floating 14-Bolt is to convert the antiquated drums to discs using Caddy-sourced rear brakes parts. But being on a budget lead Jeremy to go a slightly different direction, using the same front disc brake setup on the 8-lug K20 10-bolt he’d swapped under his K5 Blazer.

To make the conversion he turned to Skys Off Road Design for the custom brackets needed to mount the disc brakes to the 14-Bolt axle housing. (Skys’ kit only fits the Corporate 14-Bolt, full-floating, 8-lug axles with wheels studs pressed through the drum.) What stands out about SORD’s brackets is they are CNC-machined to hold .006-inch tolerances, which means they fit tight and clean, allowing the calipers work at maximum efficiency. This design also allows the brackets to be used without need of any spacers, which can put more stress on the bolts during extreme off-road use.

In addition to the brackets, a pair of new rotors and calipers that fit ’73-’87 K20 4x4s were picked up at the local auto parts store, along with a master cylinder for a 1980 Corvette and a PV4 proportioning valve from Summit Racing. The conversion process from that point on is a simple bolt-on.

The downside of this rear disc-brake conversion is there’s no longer a mechanical e-brake, so another alternative has to be found. Jeremy did what many enthusiasts have done in the past and that was to install TSM Manufacturing’s NP205 “MB2-Series” parking brake kit. It’s a simple bolt-on installation. The kit provides the mechanical braking advantage of going through the axle’s gearing to hold the truck’s weight, and with the Detroit Locker in his 14-Bolt, it’ll work like an emergency brake in most situations if the front differential is locked in.

Converting from drum to disc rear brakes is an easy performance upgrade for the 14-Bolt owner as you can see from our highlights. Disc brakes readily sling off mud and water, and they cool faster, providing better overall braking and longer life than their aged drum counterparts. They are also less expensive to maintain. Those are all win-win attributes and good reasons to make such a move to step-up your rig’s braking power.

Factory drum-style brakes on 8-lug GM Corporate 14-bolt axles are adequate for stock applications. But they lack the durability or efficiency when used with taller, heavier tires.
A master cylinder for a 1980 Corvette (#A39052), a pair of discs and calipers for a ’75-‘86 K20, a PV4 proportioning valve (#760185) from Summit Racing, and conversion brackets (#CORP-DBB-14B) from Skys Off Road Design are all that’s needed to convert the 14-Bolt to disc brakes.
Once the Corporate 14-Bolt’s axle and hub retainer are removed, the hub assembly can be pulled off. Installing discs brakes reduces rolling mass.
The old drum assembly is then removed from the axle. Drum brakes collect a lot of dirt and debris, which is one the big drawbacks to their off-road performance and durability compared to discs.
Screwing an old lugnut on the studs, then using a hammer to drive them out, is an easy way to remove them without damaging the threads.
Skys’ GM 14-Bolt disc-brake conversion bracket is precision machined to provide a precise, tight fit so the caliper functions exactly as intended. There’s no spacer required. Kit comes with the mounting hardware.
Once the hub is cleaned, the new rotor is set in place and the old studs re-installed.
Cleaning the hubs and replacing the seals and bearings while everything is out is a good practice. This axle was from a junkyard dog, so the new hardware is a big step toward a long, trouble-free run under this truck.
The hub assembly, with new bearings and paint, is slid on the axle housing and secured. Pre-load using new bearings is 25-35 in./lbs., 5-15 in./lbs. if re-suing the old bearings.
The axle shaft is installed and bolted to the hub. Specs call for 115 lbs./ft. of torque on the flange to hub bolts.
Fitting the new caliper over the rotor and bolting it to Sky’s CNC-machined adapter plate is quick and simple. Kit comes with new bolts and instructions.
We used a liberal amount of Permatex Ceramic Xtreme brake parts lubricant to lubricate the locking pin.
Ready to roll. The 14-Bolt disc conversion is clean, and it’ll provide considerably better braking power and performance in the off-road environment that it’ll be subjected to over the coming years.
A good master cylinder to use with a GM 14-Bolt disc brake setup is one for a 1980 Corvette (#MCA39052), which has the larger 3/16” brake lines. It’s a direct bolt-on to the GM truck booster. Jeremy also used an Ultra Bend brakeline kit from S.U.R.&R. (#BR-EZ316) to build his own hard lines.
Installing a PV4 brake proportioning valve (Summit #760185) helps ensure proper braking balance when rear discs are installed. This one is mounted on the cross member under the engine, close to the new master cylinder.
The final piece of the 14-Bolt disc-brake conversion is setting up a parking brake. We used the “MB3” parking brake kit from TSM Manufacturing that bolts right up to the Blazer’s NP205 transfer case without any modifications. A single brake cable runs to the foot brake in the cab. Kit costs about $300, but it’s worth the price.


Brackets: Skys Off Road Manufacturing / CORP-DBB-14B
1980 Corvette master cylinder
Brake lines by Pacific Rubber – Eugene
T-case park-brake conversion: TSM NP205 kit “MB3” model
Summit proportioning valve: PV4 760185

Sources

Summit Racing
Akron, OH
800-230-3030
http://www.summitracing.com
TSM Manufacturing
303-688-6882
http://www.tsmmfg.com
Sky’s Off-Road Design
541-736-3743
skysoffroaddesign.com
Jeremy’s Muffler & Brake, Plus
541-357-4409
www.jeremysmuffler.com

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