1969 Jeep CJ-5 Master Cylinder RebuildPosted in How To: Transmission Drivetrain on May 1, 2008
One of the most basic rebuilds on a 4x4 is a brake master cylinder. They often wear out but are simple to fix. Sure, rebuilt units can be had cheap on common vehicles, but not always for the specialized stuff we drive in the dirt. If we were young and hip (-hop), a master-cylinder rebuild story would be peppered with references to "sucka MCs" and "bumpin'" hydraulics. Too bad some of us are too old to care about "street cred." Instead, it's all about credibility off the street. Although you can go far without a functioning master cylinder, our goal here is to help you stop short of going too far.
Master-cylinder failure usually shows symptoms other than the vehicle not stopping as intended. Spongy brakes are a prominent indicator. A brake pedal that slowly sinks to the floorboard under foot pressure is a bad sign. For verification, disconnect the lines at the master cylinder's outlet ports and seal the ports with solid plugs. A firm, steady pedal means that the master cylinder is good and the problem is somewhere else in the system. A spongy pedal or one that goes all the way to the floor means that the master cylinder is bad-fluid is backflowing while pressure is being applied.
Depending on your skill level and definition of "free time," the best solution might be buying a replacement master cylinder.
A rebuilt OE unit from the local parts store is another viable option. These master cylinders are often better than factory, some using stainless-steel bore sleeves that withstand contaminated brake fluid better than the original cast surfaces. Add in a warranty and a rebuilt unit is tough to beat.
The other route is a DIY rebuild. On the surface, this is the low-buck fix. Rebuild kits typically run $20-$25. This appealed to us for several reasons: (1) replacing the piston, seals, and valves is supposedly about a 30-minute job; (2) for slow-selling part numbers, a special-order rebuild kit for our '69 CJ-5 was available the next day compared to four-day normal shipping to obtain a complete rebuilt unit; (3) cost was $22 for a rebuild kit versus $220 for our low-demand rebuilt master cylinder; (4) we're masochists.
Toolwise, the job requires a brake-hone kit, which will set you back another $25-$30. Also, a service manual is good to have on hand-the instructions in our rebuild kit dealt with bench bleeding instead of disassembly/assembly.
Major steps of a typical master-cylinder rebuild are shown here. We don't claim that you'll be "an urban legend in your own mind / Able to rhyme on a dime" after installing a funky-fresh MC, but all four of your 4x4's binders should be under pressure when powered by a properly functioning master cylinder.