Early Jeep Brake Conversions - Old SchoolPosted in How To: Transmission Drivetrain on August 29, 2006 0) (
Maybe those little 9x1.75-inch drum brakes on the MBs and GPWs were fine for the military and limited highway usage, but in reality they were marginal. The Jeep folks, for some reason, didn't get with the program and install larger brakes on the CJs until 1961 and then only added 1 inch to the diameter. At first, it was only to the special edition Tuxedo Park models. Over 10 years later in 1972, they finally decided 11 inches was the magic number for drum size.
The Wagoneers, pickups, and even the Jeepster had disc brakes by 1972, but not the CJs. It wasn't until 1977 that discs were offered as standard equipment on the CJs, but then Jeep reverted back to smaller 10-inch drums in the rear for several years.
Add some monster (for the time) 31-32-inch tires and an emergency stop became, well, darn scary with those 9-inch-diameter drum brakes. Maybe it made us better drivers because we constantly had to think ahead to judge stopping distance. Get the brakes wet and it was like, "what brakes?"
There were several different solutions tried in the late 1950s and early 1960s. One was to install a power boost unit called a Hydrovac. This usually came off something like a 2 1/2-ton truck and mounted in the line between the master cylinder and the brakes. Vacuum was used to activate a diaphragm that multiplied pressure on to a separate hydraulic piston that increased line pressure to the brake's wheel cylinders. While you gained more braking pressure, you still had the small brake drums and shoes and the brakes were touchy, to say the least.
The '49-'51 Mercurys used a Dana 44 rearend with a 5-on-5.5 wheel bolt pattern. They had huge brakes by early Jeep standards - 2 inches wide and 11 inches in diameter. As it just so happened, they were, as I remember, pretty close to a direct bolt-up. But not quite. The center hole in the drums had to be enlarged. The fronts required some modifications such as enlarging the hole in the backing plate for the larger spindle and some slight grinding. The rears required a spacer plate. The design of the brakes operation was also much better, making for not only easier adjustment but a servo action to gain shoe contact pressure.
I can still remember the Mercury that I took mine off of. It was abandoned in a vacant lot, but a pretty complete two-door. I had my eye on it for a year or so, and one day just got brave and took the brakes. Should have left the brakes on and took the whole car because they are now worth more than my Jeep.
There was a start-up company called ROCK-ETT Products that sold the complete kit with new wheel cylinders, drums, backing plates, and the spacer for about $80 in the early 1960s. ROCK-ETT went on to become what we know now as Smittybilt.
The larger brakes of the Mercury had a 1 1/8-inch-diameter wheel cylinder piston versus the 3/4-inchers of the Jeep. This presented both a problem and a benefit. Because of the larger volume necessary for the larger wheel cylinders, it took more piston stroke to meet stopping needs, so the shoes needed to be right on in adjustment. However, the added line pressure produced resulted in almost power brakes.
Another solution was to use the brakes off the Jeep utility wagon or pickup. However, these still used the cam anchor type of adjustment and weren't quite as efficient, but they were a direct bolt-on. With the introduction of the Wagoneers came brakes very similar to those on the early Mercury, but these were a direct replacement for the CJs. However, one just didn't find many Wagoneers in wrecking yards at the time.
Around 1970, Lloyd Novak found that with some modifications 1/2-ton Ford pickup brakes could also be used, and he sold an instruction sheet for a couple of bucks that told how to do this. Smart guy Lloyd was, he also figured out a way to adapt Corvette disc brakes to work on the front. However, this wasn't a particularly popular conversion because the cost and scarcity of 'vettes in wrecking yards. Maybe they were good ideas on his part, but once the instructions were in someone's hands, they got passed from club member to club member. Needless to say, he didn't sell a lot of instructions.
Remember the early standard disc brakes of 1977-78? Nice thing about this setup was the fact that the mounting bracket for the caliper would bolt right on to an earlier closed-knuckle Dana 25 or 27 frontend. Lots of us used these early smallish frontends for years, and this was a welcome conversion
We also discovered that, with some slight modifications, the disc brake setup off of the early '70s Chevy 1/2-ton pickups would bolt-up to our closed-knuckle frontends. The six-bolt wheel pattern of the Chevy presented a problem that was easily solved by using the early '77-'78 1 1/8-inch-thick Jeep or Scout II rotor.
Engine swaps often presented a problem, due to the frame-mounted master cylinder and the through-the-floor pedals of early CJs. There were a few companies such as Amquist, Honest Charlies, and later Advance Adapters that offered an overhead or hanging pedal system to utilize a separate hydraulic clutch and brake master cylinder. Advance Adapters still offers a similar conversion for early Jeeps (PN 716118).
Another trick was to modify an early '52-'53 Ford pedal assembly to fit under the dash and then use a GM truck master cylinder that in one casting utilized both clutch and brake cylinders.
During the factory V-6 years, Jeep kept the master cylinder on the frame but opted to make it a dual-piston design, as most likely by this time the dual system master cylinders were required by the feds.
With the disc front/drum rear combination, some tried to use the original single master cylinder with various results. The piston diameter and stroke length usually made for a lot of pedal travel. Plus, the residual pressure valve within the master cylinder held the pads slightly against the rotor even without one's foot on the brake pedal. Removing the pressure check valve from the master cylinder made for slow rear brake action, so a 5-7 psi valve had to be installed in the rear line. Still not a good solution with the single-piston master cylinder.
There was a company that made a conversion kit that utilized dual-piston design for early retrofit for those that wanted to keep the pedal assembly under the floor. The last set I ever saw was one that came from Willys Works in Tucson, Arizona, some 10 or more years ago.
Today you can order up a set of disc brakes for all for corners to match your custom or stock axles, as well as a disc brake emergency brake that bolts to the transfer case. There are also power boosters that work off of engine vacuum or hydraulic pressure from the power steering pump.
Yes, brakes have come a long way from those cam-adjusted 9-inchers of the early Jeeps, but the braking battle is still on with tires getting larger and vehicles getting heavier.