A faulty proportioning valve is a common cause of premature rear wheel lockup on conventional brake systems. Brake experts said bad proportioning valves cause the same problem on vehicles equipped with antilock brakes, but the resulting complaint is oversensitive or overactive ABS. Vehicle weight shifts forward suddenly during panic stops, increasing the risk of premature rear wheel lockup and then vehicle skidding. A proportioning valve enhances vehicle control during panic stops by automatically limiting brake fluid pressure to the rear brakes in these driving situations.
Brake service specialists said faulty proportioning valves often complicate ABS diagnosis by creating the sensation that the ABS activates too easily. Instead of activating only during panic stops, the ABS turns on during light-to-moderate braking. What's more, many technicians don't associate this symptom with a fundamental brake component such as the proportioning valve.
Proportioning valves, which have been used for years on both non-ABS and ABS brake systems, may be threaded directly into the master cylinder itself. On a split-diagonal braking system, there's one valve per rear brake circuit.
Or, the proportioning valve may be housed in a separate assembly mounted on the vehicle's frame.
Traditionally, technicians have diagnosed faulty proportioning valves by a process of elimination. Namely, they blamed the proportioning valve when they could find no other causes of premature rear-wheel lockup conditions.
According to Eric Lang, a brake technical specialist with EIS Brake Parts, most proportioning valves are easy to diagnose with a brake pressure test kit. Most kits package a pair of pressure gauges as well as an assortment of brake hydraulic adapter fittings.
Connect one pressure gauge to the inlet or master-cylinder side of the proportioning valve. Depending upon brake system design, it may be easiest to tee the gauge into the rear brake circuit right at the master cylinder itself. Hydraulically speaking, threading the gauge into a master cylinder or front caliper bleeder port also may be an accurate, convenient test point, according to Mr. Lang. Then connect another pressure gauge to the proportioning valve outlet-the port that feeds the rear brake circuit. Start the engine, apply the brake pedal with moderate to firm pressure and watch the reactions on the pressure gauges.
Some vehicle manufacturers provide specifications for proportioning valve tests; most do not. When specs aren't available, Mr. Lang recommends using the following procedure: First, applying the brake pedal firmly should make both pressure gauge readings increase equally and steadily up to the ``split'' point.
The split point, which usually occurs between 200-600 psi, is where a proportioning valve begins modulating pressure to the rear brake circuit. Above the split point, input pressure (the front gauge reading) should continue increasing. The output pressure (the rear gauge reading) may remain stable above the split point. More likely, output pressure will continue increasing above the split point, but at a much slower rate than input pressure.
A good rule of thumb is that with the brake pedal firmly applied, proportioning valve output pressure should not exceed half to two-thirds the input pressure. Replace the proportioning valve if output pressures climb beyond specs or these working limits.
In other words, if input pressure peaks at 500 psi, proportioning valve output pressure should not exceed 250-330 psi. Technicians who practice the pressure gauge technique quickly learn the telltale reactions of faulty proportioning valves, he added.
When the brake fluid has been neglected, dirt and sludge may form in the hydraulic system, restricting or jamming the proportioning valve. The result is unusually low proportioning valve outlet pressure or none at all. Consequently, the front brakes wear out prematurely and repeatedly because the rears are doing little or no work.
Plus, technicians must individually test the proportioning valves on split diagonal braking systems. Of course, a bad proportioning valve here could mean premature lockup or inadequate braking on just one rear wheel.