Almost a brake failure!

[wally888]

Recently there has been some discussion about a newly identified symtom of sick brakes, possibly just before a failure or......<BR> I, and a few others, have noticed that sometimes when applying the pedal, there is a slight hangup, resistance, catch and release feel( hard to describe but not what you normally feel).<BR> Yesterday, while returning from a trip to Atlanta, just prior to turning onto the interstate, going about 6 mph, as I gently braked to stop behind a truck also turning, I experienced this hangup and a momentary rise in the pedal( got hard, stiff but not rock hard)and I felt a Servo in the Valve Block cycle 3 or 4 times( all this happened in less than 2 seconds). By the time I said oops, everything was back to normal.<BR> Had the problem continued I might have found myself w/ a hard pedal and no brakes other than manual. A simular situation to several previously described just before wrecks, near misses or hard pedal and no hydraulic assist and NO LIGHT WARNING.<BR> I will check the ALDL for codes as this appears to be a malfunction of Antilock. Something told the EBCM that a wheel was locking, the EBCM sent a signal to open a Servo to relieve pressure in that caliper, the EBCM signal also opened the Main Valve allowing pressure to enter the normally unpressurized portion of the Master Cylinder, which in turn caused the Reaction Sleeve to raise the pedal. <BR> Had all this continued and had I been going faster, who knows, but maybe, 1. continued hard pedal and only manual braking? 2. a wheel lock or too much braking on the opposite caliper? 3. no braking on the affected side?<BR> Of course the question here is what caused this to happen. I have no answer, have some possibilities, most off the wall. Would appreciate speculation from Barney and others. Did a wheel almost lock, why, if so, then everything electrical was normal. Did a calipers failure to back off cause the wheel locking signal? Something opened the Servo, probably a signal , legitimate? Bottom line-was this mechanical or electronic?<BR> I have submitted a list of questions to the Antilock expert at Prior Rebuilders, he has agreed to provide answers. The situation above is described in one of the questions. The questions are slanted in a way to help us determine what happens when, i. e., The Main Valve fails, a servo fails, one or both seals in the Master Cylinder fails,...<BR> With his answers and suggestions, I hope we will be on a path to taking the necessary steps to eliminate most future failures.

[bigrog]

Sound like premature lockup detected by the ABS control unit. Usually caused by faulty wheel speed sensor input. Can be caused by a faulty wheel speed sensor, or a poor connection in that circuit. Agressive driving should make a trouble code set, and then you will know which wheel speed circuit to inspect. Pretty normal ABS symptom on any car.

[Richard S]

While bigrog's reply makes sense, I can only say that in Allantes pedal "kickback" of the kind Wally describes is almost always associated with a failing accumulator. Would be nice if cleaning up or checking the connections at a wheel sensor resolved the problem. When it comes time to deal with the ABS system, I am, very unfortunately, convinced that a complete rebuild, as opposed to component replacement, is the only viable solution.<BR>

[wally888]

Pulled codes from ALDL, one, history, #47-both rear sensors. Have no idea how long it has been there.<BR> I have composed a list of questions for a remanufacturer, asked them, and received answers. This problem was one of the several questions. <BR> If you would like the list, email me.

I, too, had problems with the "hard" brake pedal syndrome. Initially received ABS light signal failure. Followed advice received here and replaced accumulator and 3 wheel speed sensors. No problems since then.<BR>L.J. Frye