Mechanics

BRAKE PEDAL PULSATING

1997 Mercury Marquis • 80,000 miles

I was stopping at a stop sign and was moving about 2 miles per hour when my brake pedal started pulsating real fast and the pedal went further down than normal. I still had some brakes. I took my foot off the pedal after about 10 seconds and pushed the pedal again and the brakes were back to normal. It has only happened the one time. I have no idea where to start looking for the problem. My pads and rotors are ok. Thank you for any help you might have.
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Buckster
May 30, 2013.



Rapid pulsing at that speed means the car has anti-lock brakes. Most likely your system has a pump that builds up and stores brake fluid under pressure. That is needed when the system activates to apply and release the brake at one wheel, and many systems use that pressurized fluid to provide the power assist. Each time the pedal is pressed some of that pressure is lost, then the pump runs to build it back up. If that's how your system works you can prove it by pumping the brake pedal repeatedly while the car is standing still. After perhaps a dozen pumps you'll hear and feel the pump kick in but you may have to listen closely.

I have a real effective system on my '93 Dodge Dynasty that works this way. It has many advantages over standard power brakes that run on engine vacuum.

It is also possible the anti-lock function was kicking in because the computer thought one wheel was locking up. Most anti-lock systems do not operate below about three miles per hour and you were right at that threshold. The computer may have seen three wheel speed sensors still producing a signal and one that wasn't. The pedal pulsation would be a secondary result of the computer turning that one brake on and off real fast in an attempt to get the wheel speed back up to the same as the others. That is normal operation.

Caradiodoc
May 30, 2013.