4x4 Disc Brakes - Valving Variables - 4Wheel & Off-Road Magazine

Bench-bleeding a rebuilt or replacement master cylinder is easier and faster than attempting to purge air with the unit on the vehicle. This cheapie parts-store kit feeds fluid from the ports back into the reservoir as the piston is plunged with a blunt object. Once all air bubbles disappear from the master's ports and the bleeder kit's tubing, promptly put the lid on the master, install it on the vehicle, and bleed the brakes to minimize additional air from entering the system.

Bench-bleeding a rebuilt or replacement master cylinder is easier and faster than attempti

Once the master cylinder and pedal ratio (if adjustable) are mathematically matched to the calipers' fluid-displacement requirements, hydraulic valves are used to tailor the pressure and volume. Proportioning-front/rear biasing-is necessary because the vehicle's leading end needs more braking power. The other end (i.e., rear) actually needs to grab first to avoid an unsafe nose-dive, analogous to the front brake throwing a cyclist over the handlebars. However, the front rate of rise is higher, meaning that the rears actuate first but grab more slowly than the fronts. OE systems typically use a combination or metering valve that regulates front/rear pressure in addition to containing a warning-light switch. Consult the brake specialists listed here to see if an OE-application valve will work for your conversion.

For the rest of us, an aftermarket adjustable proportioning valve allows the rear brakes to be fine-tuned. If they lock up before the fronts in a panic stop, the adjustable proportioning valve will help limit the rear rate of rise. Ballpark pressure for rear calipers is around 750 psi. This can be used as a starting point, then the proportioning valve can be adjusted a quarter turn at a time until braking passes the seat-of-the-pants test on a long, deserted road with no poles alongside it: In a panic stop, both ends should lock around the same time.

In disc systems, 2-psi residual pressure valves maintain a preload on the calipers' pistons so that they won't retract fully (quicker activation). This typically happens when the calipers are located above the master cylinder and fluid siphons back into the reservoir. If the pedal needs two pumps for the pads to grab, 2-psi residual valves can be a cure.

One of the Camaro MC's ports is larger than the corresponding CJ's. This requires an adapter fitting if using the Camaro master with Jeep-spec lines.
One of the Camaro MC's ports is larger than the corresponding CJ's. This requires an adapt
DOT 3 brake fluid is hydroscopic-it absorbs condensation and "breaks down" over time. Accumulated sludge pitted this master cylinder's bore, allowing fluid to back-flow with the pedal depressed.
DOT 3 brake fluid is hydroscopic-it absorbs condensation and "breaks down" over time. Accu
If using a replacement-line kit, begin by comparing the new lines to the existing ones. Older 4x4s often have nonstock routing resulting from suspension mods such as shackle reversals.
If using a replacement-line kit, begin by comparing the new lines to the existing ones. Ol

Old brake lines are often "rust-welded." Spray all fittings with penetrating lubricant before attempting to break them loose with a line wrench.
Old brake lines are often "rust-welded." Spray all fittings with penetrating lubricant bef
If the old nuts absolutely won't budge and/or their shoulders get rounded, try Vise-Grips. Some aftermarket brake specialists suggest cracking a bleeder and using a butane torch to heat the fitting to break it loose, but realize that brake fluid is flammable before choosing this method.
If the old nuts absolutely won't budge and/or their shoulders get rounded, try Vise-Grips.
As a last resort, cut the line as close to the fitting as possible. Always save the old lines for comparison to their replacements or to use as patterns. Large lines can be shipped to mail-order line specialists in a bicycle box to be duplicated.
As a last resort, cut the line as close to the fitting as possible. Always save the old li

Power-assists: These units can solve low-pressure problems. However, vacuum boosters require a minimum of 16 inches of engine vacuum as well as space for mounting the booster. (The larger the booster, the more assist it provides.) Remote-mounted boosters and even electric vacuum pumps are marketed to hot-rodders, as are other potential fixes such as ECI's frame-mounted Brake Pressure Multiplier. Another possible low-pressure solution is a steering-pump-driven hydroboost system such as the one from Vanco.

Hoses: Lift kits sometimes come complete with longer brake hoses; suspension downtravel can cause too-short hoses to kink, either locking fluid in the caliper or preventing fluid from entering. The worst-case scenario is a snapped hose. Rubber hoses are mainstays, but many brands of braided-steel hoses are now DOT-compliant. They're more expensive but offer better durability and pedal feel.

Fluid: DOT 3 and DOT 4 glycol-based fluid works fine for most recreational applications. DOT 5 silicone fluid doesn't absorb water or eat paint, but it isn't compatible with glycol-based fluids and can compress under pressure. Most nonracers don't feel that DOT 5's advantages are worth thoroughly flushing the system. Glycol brake fluids will absorb ambient moisture, so always use a new bottle for best results. Along the same lines, thoroughly flush your 4x4's glycol brake fluid every year or two (depending on usage and conditions) to keep it from turning to sludge.

Leaks: Never use Teflon tape or pipe dope to seal brake lines. The seal is actually made at the lines' flares, not by the nut's threads. Inline Tube says that most leaks result from improper seating and recommends loosening and tightening the fitting several times so the line's flare can form a leakproof seat.