A brake pedal that occasionally drops to the floor suggests hydraulic brake trouble to many people. But this sporadic condition, which misleads many techs, really is caused by mechanical problems outside the brake system itself, brake specialists said. ``Knockback'' occurs when something unexpectedly pushes a caliper piston deep back into its bore, forcing extra fluid back into the master cylinder at the same time. The next time the driver applies the brakes, the brake pedal falls momentarily but returns to its regular height. The reason the pedal drops once is that it takes one extra-long stroke to displace the extra fluid, pushing the piston back to its previous position in the caliper bore. With the piston repositioned and the fluid replenished, the brake pedal feels normal again.
When troubleshooting a sporadic low-brake-pedal complaint, always check pedal feel with the steering wheel turned to each lock, advised Joe Meyer, an instructor at Professional Automotive Career Training (PACT), a division of Conrad's Goodyear, Cleveland.
If the brake pedal only falls to the floor with the steering wheel turned to the lock, check for a misadjusted, worn or missing steering stop.
The steering stop is a bolt, bracket or rubber piece that's usually attached to the lower control arm and prevents the steering knuckle from pivoting too far during turns.
When this stop is misadjusted, worn or missing, it may allow the brake caliper to strike the frame during a tight turn, temporarily forcing the caliper piston deep into its bore. The next time the driver applies the brakes, he notices excessive pedal travel just once-then the pedal feels fine again.
A sloppy wheel bearing also can cause caliper piston knockback. Usually, a wheel bearing in this condition is also creating handling problems and making noise.
Mr. Meyer emphasized that piston knockback has fooled some technicians. However, its causes should be obvious to alert technicians performing thorough undercar inspections.