 | ||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||
|
The Fading Bronco Brake PedalPosted 12/9/1999By Jeff Boskowitz
Vehicles: 1987 through 1998 Ford Bronco. All engines with Rear Anti-lock Braking Systems (RABS). System: Brakes/ABS. Symptom/Problem: Brake pedal fade, low pedal. The problem is an internal leak in RABS valve. Background: A common complaint on Ford light duty trucks with the rear anti-lock brake system is that the brake pedal will fade while under steady pressure. In a conventional brake system this would point to an external fluid leak or an internal leak (bypass) in the master cylinder. Generally, one and usually two master cylinders have been replaced already, leaving one very frustrated technician. In this system there is one other possibility - the rear anti-lock brake valve. This valve is the electro-hydraulic proportioning valve located on the left-hand frame rail between the master cylinder and the rear brakes. This valve controls the brake fluid during an anti-lock stop by restricting or bleeding off rear wheel cylinder pressure. To better understand what's happening, let's take a quick look at system operation. The RABS module continuously monitors rear wheel speed by means of a sensor installed in the rear differential. When the sensor signal to the module shows a rapid decrease in speed - indicating rear wheel lockup will occur - it energizes the isolation solenoid in the RABS valve. By doing this, the normally open valve is now closed, blocking off the line to prevent further brake fluid from entering the rear wheel cylinders. If there is still too great a decrease in rear wheel speed, the module will then energize the dump solenoid in the RABS valve. This solenoid is normally closed and, when energized, will open a port to the accumulator and bleed off some of the pressure to prevent lockup. This fluid is then stored until normal brake functions are resumed and the fluid is returned to the master cylinder. If rear wheel speed is now OK, the module de-energizes these solenoids to resume normal braking. If speed decreases too rapidly again, it resumes anti-lock control. The module will alternate between the two solenoids and normal braking until there is no further chance of locking up or vehicle speed is now under 5 mph. The focus is on this dump portion of the RABS valve. Occasionally the seat between the solenoid and the accumulator will leak, causing the pedal to fade during normal pedal applications. This sinking pedal has no other abnormal braking characteristics and is usually not noticed (depending on severity) unless it sits for an extended amount of time with constant pressure applied. It may be due to physical damage or a buildup of contaminants in the hydraulic system, but the results are the same. Some customers report that the brakes were just fine until a hydraulic component was replaced or repaired and the brakes were bled. Then the pedal started fading. Quite possibly this is due to a new problem that is created by "moving" some of the contaminated fluid into the valve. To diagnose this system, like all brake systems, first verify that there are no external fluid leaks. If visual inspection is OK, disconnect the brake lines at the master cylinder and plug the outlets in the cylinder. When the master cylinder is plugged, if a fade is still experienced, the master is at fault. If, however, there is no fade, determine if it is coming from the front or rear brakes by blocking one port at a time at the master cylinder. If there is no longer a fading pedal when blocking the port to the rear line, go one step further and build a short brake line to splice between the inlet and outlet lines at the RABS valve to temporarily bypass the valve. After bleeding the rear brakes and verifying there is a solid pedal, it's a good bet that the problem is in the valve itself. The next step is to replace and bleed the valve (if equipped with a bleeder) and the rear wheel cylinders. Verify the repair and road test.
|
|||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | ||||||||||||||||||||||||||||||||||
This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.
