The federal mandate for a dual master cylinder ensures that if the brakes fail at one axle, the other axle's hydraulics remain functional. Such a failure requires an immediate warning to the driver. Pressure lost to one hydraulic circuit shifts a valve over and activates a warning light on the dashboard. The switch, which mounts between the two brake lines leaving the master cylinder, is the pressure differential brake warning switch.

Engineers took the concept a step further. The "combination valve" addresses three concerns: One is the safety warning for a hydraulic circuit failure; another would be the proportioning of fluid pressure front to rear. When the brake pedal applies, and fluid pressure increases in the lines, the proportioning valve restricts hydraulic pressure to the rear wheels. The restricted ratio is an amount determined by the vehicle's design, weight distribution, and chassis dynamics. The proportioning valve compensates for weight bias. The last function of a combination valve is metering. A known issue with disc front and drum rear brakes is the lag time for the rear shoes to push through the return spring pressure and shoe-to-drum clearance before applying shoe pressure at the drum. By contrast, disc pads have no return springs and ride at the sides of the rotor. Lag time is virtually non-existent if the rotors run true

A metering valve delays hydraulic pressure to the front disc brakes. The aim is to synchronize the application of the front and rear brakes. For slick road surfaces and light braking, the metering valve helps prevent front-wheel lock up. Hard braking will push through this metering and deliver immediate front brake pressure.

If a disc brake changeover involves use of OEM components from a donor vehicle, the donor's combination valve may work well. The swap should include the master cylinder, combination valve, and complete wheel brake system from that donor vehicle. The "may work well" factor depends upon whether the donor vehicle and your truck have similar weight bias, center of gravity, roll center, and curb weight.

When installing an aftermarket disc brake kit, proportioning is important. Most kits include a manual proportioning valve. Use care adjusting the manual proportioning valve. Follow the manufacturer's directions to avoid a rear brake lockup that could cause the vehicle to spin out. If hauling in the bed, find a setting that works for the vehicle under various load and weight transfer conditions. Perform brake tests away from traffic and obstacles.

Note: There are knob and lever-actuated manual proportioning valves. One lever type uses a six-position notch lock handle. Mounted within reach, the valve can be adjusted quickly to compensate for changing track or road conditions, off-road driving terrain, bed loads, and so forth.

Disc Brake Service Needs
Unless the rotor needs surfacing, pad and caliper work can be performed without removing the wheel hub. Many brake and repair shops now have equipment for resurfacing rotors on the vehicle with the hubs installed.

When working on a brake system, brake dust can be hazardous. The truck might be old enough to have asbestos content lining, or asbestos type could have been installed during service work. (Don't assume that if a vehicle is new enough, the lining is non-asbestos.) Use high-quality, non-asbestos replacement lining.

Brake parts cleaners with petroleum solvents or distillates are harmful to rubber parts. Avoid spraying rubber parts like the brake hose or seal around the caliper piston. Wash down metal parts before and after removing the caliper. Keep the dust in solution, and dispose of it in a safe manner. Dry, "friable" asbestos is a hazardous material.