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GM Medium Duty Truck Brake Booster Upgrade

 

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GMC 6000 and Chevy C60 trucks often have a firewall mounted master cylinder that feeds a frame mounted slave cylinder and brake booster. These trucks had a single circuit system feeding all 4 drums. They worked ok but replacement parts are nearly impossible to source. My solution is to place a modern Ford Super Duty 11" dual diaphragm booster and large 1.5" bore master cylinder on the firewall with fabricated mounted brackets. Super Duty boosters are easier to source, and the Ford boosters are nearly the same design as GM parts. The booster setup in these kits is the same as most newer F550s would get from the factory.

These applications require a longer than normal rod length from booster to eyelet that connects to the pedal. This is part because of the factory design and part because a set of booster brackets hold it further away from the firewall. There isn't any off the shelf booster that has a long enough rod so we cut the SD booster rod, insert each end into a machined spacer, and tig weld it back together. The spacer is solid in the middle, so the machined rod does the holding and the welds don't have to support that force.

These boosters accept either 1 3/8" bore or 1 1/2" bore master cylinders. I expect that the larger 1.5" bore master is the way to go. Larger bore masters give a better pedal feel, although they do slightly reduce the fluid pressure. These systems still put out around 1,500 psi with the 1.5" bore master so they'll be plenty powerful.

 

 

1967+ C60 / 6000 Original  System

This info is from a 1976 GMC 6000. If your truck is different please let me know and I can update the info here.
The firewall mounting setup of the original brake slave cylinder is a simple 2 bolt pattern, square over the firewall opening. The bolt holes are 3 5/8" on center. The distance from the back of the slave cylinder (where it touches the firewall) to the center of the eyelet connecting to the brake pedal is about 8 5/8". These are the main measurements to double check to confirm fitment.
Underneath the frame you can find the junction where the oem master cylinder fed into with one line, and 2 lines exit for the front brakes, and 1 line exits for the rear brakes. You'll want to separate out these circuits and add a proportioning valve.

 

 
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1961-66 C60 / 6000 Original  System

The earlier trucks use a 2 in 1 clutch/brake slave system with this double bore master mounted on the firewall. It is held on by 3 bolts. The clutch system is actually on the passenger side, brakes on driver side. 

 
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Firewall Mounted Modern Booster

A modern 11" dual diaphragm booster can be mounted in place of the original master cylinder with special made steel brackets, a tig welded booster rod extension, and a machined bushing to connect the booster rod to the pedal. This way, the new booster has all of the same mounting specs as the original master cylinder. The booster rod spacer is solid in the middle, and hollow for a depth of 1" on each end. This ensures the rod stays square during welding and also means that the welds are not holding the force from the pedal.
The booster has a 3/8" barb inlet

The master cylinder has 12mm bubble flare and 10mm bubble flare ports. These can be adapted to 7/16 flare fittings with Edelmann 262800 and 262500, which are included. 1/4" brake lines commonly have 7/16 threaded fittings.
 

 
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Firewall Booster Installed
 

 
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