Time to Put the Brakes on a Serious Problem
Question: I am having brake problems on my 1991 Mercury Grand Marquis. The back wheels lock up when I apply the brakes. So far, the dealership sanded the front disc pads on the first visit, adjusted a rod in the master cylinder on the second and replaced the rear wheel cylinders on the third. Although there seems to be a slight improvement, they still seem to lock up fast and the car skids to a halt. Help.
--G.C.
Answer: You have a potentially serious safety problem that needs expert attention. Theoretically, all four wheels on any car should lock up simultaneously. You obviously don’t want any one brake locking up before the others, because the vehicle could be thrown into an uncontrolled skid.
The brake system is supposed to be designed with just the right amount of braking power at each wheel to ensure simultaneous lock-up. But sometimes one brake surface can get greasy or not get adequate hydraulic pressure and the other wheels will lock up first.
The proposed fixes by the dealer made a little bit of sense, but they are sort of things that a back-yard mechanic might try. I figure a good mechanic should have zeroed in on a potential defect in the brake proportioning valve, a very important part of the braking system.
The proportioning valve determines how much hydraulic pressure is allocated to the front versus the rear brakes. It is not a very sophisticated part. It is a small block of machined steel with tiny passageways drilled through it, so that the correct amount of brake fluid flows to the rear and the front brakes.
Even though there is more weight in the front of a car than the rear, and the size of the brakes differ, the proportion valve is supposed to even things out. It’s possible in your case that the proportioning valve is allowing too much hydraulic pressure to go to the rear wheels.
The proportioning valve also acts as a warning sensor for a hydraulic system failure. Under federal law, all brake systems have two circuits so that if a loss of hydraulic pressure occurs at one wheel you still get braking in at least other two wheels. The proportioning valve detects failures and triggers a warning light on the dashboard.
Another possible cause of your problem is a defective brake master cylinder, which is a sort of hydraulic pump. Possibly, a defective internal seal is allowing pressure to bypass the chamber that supplies pressure to the front brakes and add pressure to the chamber that serves the rear brakes.
You might have avoided all of this fuss if you had anti-lock brakes, which are designed not to lock up. On anti-lock brakes, a sensor on each wheel determines whether it has locked up and automatically reduces pressure to that wheel.
It causes a rapid pumping action that prevents prolonged skids. The system is designed to stop a car more quickly, particularly on wet or icy surfaces.
As a side benefit, the anti-lock system will tend to compensate for a number of general brake problems that may develop or for a poorly designed system. Of course, the drawback is that the anti-lock system also can disguise a problem that should be fixed.
Nobody should put up with defective brakes on a car still under factory warranty. Tell the dealer to find the problem quick, because he is running out of time if he wants to avoid a Lemon Law suit.
