When you push down on your brake pedal, hydraulic pressure from the master cylinder is routed to all four wheels to apply the brakes. The front brakes usually receive more pressure than the rear brakes for two reasons. The front brakes receive more weight transfer when braking hard (and thus require more braking effort). The rear brakes usually carry less load, so pressure must be reduced to prevent them from locking up and skidding when braking hard.
BRAKE PROPORTIONING VALVE
The relative brake balance front-to-rear on most older vehicles is controlled by a hydraulic Brake Proportioning Valve mounted on or near the master cylinder. The proportioning valve reduces pressure to the rear brakes by 20 to 40 percent depending on the application.
In vehicles like vans and pickup trucks that are typically nose-heavy, a load-sensing proportioning valve is often used that shifts more brake force to the rear wheels when the cargo area is heavily loaded. Accurate adjustment of the load-sensing valve is essential for proper brake balance.
HOW BRAKE BALANCE AFFECTS BRAKE WEAR
In vehicles with a mechanical hydraulic proportioning valve, the front brakes work harder and typically wear at a rate two to three times faster than the rear brake pads or shoes. So historically, rear brake linings have typically lasted two to three times longer than the front linings.
ELECTRONIC BRAKE PROPORTIONING
Electronic brake proportioning (also called Electronic Brake Distribution or EBD) was introduced a number of years ago, and is now becoming common on many late model import and domestic vehicles. Some applications include late model Honda, Toyota, Audi, Mercedes and others. The Honda Accord was equipped with electronic brake proportioning in 2006, and all Honda vehicles, except the S2000, now have it.
With electronic brake proportioning, pressure from the master cylinder is routed equally to the front and rear brakes. There is no mechanical proportioning valve in the brake system. Instead, the Antilock Brake System (ABS) controls what happens at the rear brakes.
The ABS system monitors the speed of the rear wheels via the wheel speed sensors (WSS) when braking. As long as the rear wheels are slowing at the same rate as the ones up front, and there is no indication that the rear brakes are locking up, full braking force is applied to the rear wheels. With the rear brakes now doing 50% of the work (instead of 20 to 40%), the vehicle stops quicker and in less distance.
On the other hand, if the rear wheels are starting to lock up because the vehicle is braking hard on a wet or slick surface, or because the rear wheels are lightly loaded, the ABS system intervenes to reduce pressure to the rear wheels. This is done by closing solenoids in the ABS hydraulic unit that isolate the rear brake circuits. Other solenoids are then opened to release pressure from those circuits. The solenoids are then repositioned so pressure can be reapplied to the rear brakes.
HOW ELECTRONIC BRAKE PROPORTIONING AFFECTS BRAKE WEAR
On vehicles with electronic brake proportioning, the REAR brakes may actually wear out faster than the front brakes. This is normal, though annoying if it occurs at a relatively low mileage. Some vehicles with electronic brake proportioning have had problems with the rear brakes wearing prematurely (say 25,000 to 30,000 miles). The fix has been to replace the rear brake pads with ones that are harder and more wear resistant.
On these applications, the front brakes may still have half or more of their normal service life remaining when the rear brakes have to be replaced. This does not mean you can ignore the front brakes. The front brakes should also be inspected and replaced as needed depending on the condition of the pads, rotors and calipers. If your front brakes are still in good condition with adequate pad thickness left, only the rear pads need to be replaced.
DIAGNOSING ABNORMAL BRAKE WEAR
Though the rear brakes may wear faster than the front brakes on some late model vehicles, unusually rapid brake wear front or rear, or uneven brake wear at any wheel may indicate trouble. Brake noise, pedal pulsations, grabbing or fluid leaks are also symptoms that often indicate a need for brake work.
When the brakes are applied, the caliper pistons move out to squeeze the pads against the rotors. When the brakes are released, the pistons should retract allowing the pads to move away from the rotors. If a caliper piston sticks, it can cause a pad to drag against the rotor causing it to wear prematurely.
A similar thing can happen if a floating caliper sticks or binds on its slides or bushings. This prevents the caliper from centering itself over the rotor when the brakes are applied and released. The result is typically uneven pad wear. Lubricating the caliper slides and bushings with high temperature brake grease, and/or replacing the caliper slides or bushings may be necessary to cure this problem. Be sure to inspect the caliper contact points, too. If they are worn, the caliper should also be replaced.
DIAGNOSING BRAKE PROPORTIONING PROBLEMS
On a vehicle that has a conventional mechanical proportioning valve, rear wheel
lockup or skidding, activation of the ABS system under normal braking, and/or rapid rear brake wear could all indicate a problem with the proportioning valve. Inspect the rear brakes for a dragging pad or shoe. If no problems are found, the problem is likely too much pressure being routed to the rear brakes by a defective proportioning valve.
If your vehicle is taking longer than normal to stop (increased braking distance), and the rear brakes do not seem to be doing much, the proportioning valve could be reducing pressure to the rear brakes too much. Other possibilities include fluid leaks in the rear brake circuit, or fluid leaking from a brake caliper or wheel cylinder contaminating the rear linings.
On vehicles with electronic brake proportioning, loss of a wheel speed sensor signal, an erratic WSS signal, or a fault with any of the ABS control solenoids could upset the operation of the ABS system, triggering a fault code and turning on the ABS warning light. If the ABS warning light is on, electronic brake proportioning as well as ABS are temporarily disabled until you can have the fault diagnosed and repaired. A scan tool with ABS capability for your vehicle is required for diagnostics.
REAR BRAKE SERVICE TIPS
Rear calipers often need the pistons reset when replacing the pads, since the parking brake may be integral with the caliper. This may require a special tool to rotate the caliper piston.
Some late model vehicles also have electronic rear parking brakes that automatically lock the rear wheels when the transmission is placed into park. Though this causes no extra wear on the rear brakes under normal circumstances, it may accelerate rear brake wear if the electronic parking brake fails to release.