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.
Diagnosis
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.
DIY Or DIFM?
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.
Master-cylinder rebuild kits are fairly basic. They can include a piston assembly, seal cups, and valves. Since this kit is for a manual-braked CJ, the rebuild kit includes a pushrod boot and a snap ring. Service manuals are handy references, even for seasoned rebuild veterans. Details such as rubber-cup orientation often aren't addressed in the rebuild kit's instructions.
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.
Refer to the shop manual for how to remove the master cylinder from the vehicle and pull the pushrod out. Since dirt and water are the nemesis of hydraulic systems, brake fluid had absorbed condensation and turned to sludge, making this primary piston a second-class citizen. Complete the master-cylinder disassembly by removing the secondary piston, piston stop, tube seats, and residual valves. | 
Accumulated gunk also plugged the reservoirs' ports and pitted the bore so that the pistons' cups wouldn't seal. Thorough sanitizing with brake cleaner can turn the 30-minute job into a worst-case two-hour affair. Proper master cylinder reconditioning is critical to long life and good braking. | 
Even after the reservoirs were desludged, the fluid ports were still clogged. A paper clip and other various-diameter wires were tried until the perfect hole-hogging size was found. Make sure to use brake fluid as a lubricant, not oil or water. |
Use spankin' fresh brake fluid as a lubricant while honing the cylinder with the specialized tool. Hone until the cylinder wall passes both the visual and finger tests for smoothness. Badly etched cylinders require much honing, making you wish you'd opted for a professionally rebuilt master cylinder. | 
This rebuild kit includes new rubber cups for the secondary piston. For this application, the cups' flat sides face each other. With a new spring and cups, the freshened secondary piston is inserted in the honed-to-perfection bore, and the new primary pistons (included in the kit) slide into the bore after the secondary one. | 
With the retaining clip and pushrod in place, the rebuilt master cylinder can be bench-bled to minimize air intrusion into the system. This minimizes on-vehicle bleeding time which can be a pain on older 4x4s. |