His Foot’s Not on the Brakes, So What Is?
Question: The brakes on my 1990 Ford Taurus occasionally come on even when I don’t press the brake pedal. It has happened half a dozen times, but never when my mechanic or the dealer has the car.
I know for certain that the brakes are coming on because the car will noticeably slow down or even stop going downhill. I can also smell burning rubber. I believe the antilock system is malfunctioning, but Ford insists that is not possible. What can you tell me?
--M.K.
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Answer: The condition you describe is not caused by the antilock brake system, because it is not mechanically capable of applying the brakes when your foot is not on the pedal.
Rather, the problem is probably associated with something called the combination valve, which has a metering valve and a proportioning valve that ensure the stopping action in the front wheels is the same as in the rear wheels.
The Taurus, like many cars, has disc brakes in the front and drum brakes in the rear. The metering valve ensures that the rear brakes come on first and the proportioning valve ensures that the rear brakes get less hydraulic pressure. Without these valves, the rear brakes would immediately lock up in a fast stop, throwing the car into a skid.
The valves contain various springs and pistons, which can wear out or become jammed, thereby holding pressure inside the brake lines and keeping the brakes on even when your foot is off the pedal.
Another little known cause of the condition you describe is worn out flexible brake lines that are used near the wheels, says Sam Memmolo, a master technician who consults for brake manufacturers.
The rubber lines can become swollen and constricted, allowing brake fluid under high pressure to pass to the wheels but not allowing the pressure to escape back up the line after you pull your foot of the pedal.
The problem often becomes noticeable only when the brakes become hot, typically after a long drive in which the brakes get a lot of use. Sometimes, the car will also pull left or right with this problem.
Since you say the problem is intermittent, I would check into the brake lines first. They will be cheaper to inspect and replace than the combination valve.
Another rare possibility is that a vent on the power brake booster has become plugged, preventing the booster from releasing the brake even after your foot has come off the pedal.
* Vartabedian cannot answer mail personally but will attempt to respond in this column to questions of general interest. Do not telephone. Write to Your Wheels, 1875 I St. N.W. #1100, Washington, DC 20006.
