Three things can cause the warning light to turn on. All of them should be taken care of by the people who did the brake work. That's not to say you have the right to be angry with them unless they don't take care of it. Sometimes problems show up later. Most shops would rather have you come back so they can fix it at no charge rather than have you just complain to your friends and coworkers without giving them the chance to make it right.
The easiest thing to check is the brake fluid level. Three things can cause the fluid to be low. Service to the hydraulic system will allow fluid to run out. The level should have been checked before they let you have the car back. A leak will cause a low, mushy pedal. Again, the fluid must be refilled after the service is done. The third cause is worn front disc brake pads. As the linings wear, the pistons move out of their housings as a self-adjusting function. Fluid fills the large space behind the pistons. When new pads are installed, it is common practice to just push the pistons back into their housings. That forces all the fluid back up into the reservoir. For that reason, smart mechanics will not top off your brake fluid as part of a normal maintenance oil change. They know that during your next brake job, that fluid has to go somewhere, and if the reservoir is full, the extra fluid will overflow and remove any paint it comes in contact with. A lot of uninformed owners accuse their mechanics of doing shoddy work when they see their brake fluid wasn't filled. The reservoir SHOULD be full after new front brake pads are installed. If the fluid is low because the front pads are worn, that should have been caught during the inspection. Also, there is enough fluid in the reservoir to handle the needs of the front calipers without turning on the low fluid switch.
The second thing to check is the parking brake. Pull the pedal up with your hand. (People with a lever between the seats, push the handle down). If the light goes off, there is too much slack in the cables or one of the cables is sticking partially applied. The mechanic will look for evidence of a sticking cable during the inspection and service. This is one of the many areas overlooked by do-it-yourselfers. Also, if it appears to your mechanic that the cables are working properly, he probably tested their operation after the wheels were installed. A cable could stick at that point without him being able to see it. The parking brake pedal might not have fallen down far enough to turn on the light until after you drove over a bunch of rough roads. Here again, a reputable shop wants your car back to adjust the cables or look for signs of sticking. Parking brake cables should never be lubricated as this is not a permanent fix. They must be replaced. It would not be unfair if they asked you to pay for cable replacement since this isn't part of a basic, standard brake job, however, a lot of shop owners will offer to do the work at no charge for labor if you pay for parts. Its a customer satisfaction thing. They will be more likely to do something along those lines if you are a regular customer and they want you to keep coming back. To insist they provide the parts for free is not realistic or fair. If you weren't having trouble with the parking brakes before the brake service, and you didn't normally use them, there's a good chance they aren't causing the problem now.
The third thing that will cause the warning light to turn on is the "pressure differential switch". Basically it is just a simple valve and switch that compares the brake fluid pressure in the two hydraulic circuits. If a leak develops in one system, you still have the other system for safe stopping. Each system operates one front and the opposite rear wheel. This is called a "split-diagonal" system and is commonly used on front wheel drive cars. If a leak develops in one circuit, it can't build any pressure. The pressure in the other circuit moves the valve which trips the switch to turn on the warning light that tells you, "Uhm, I have a problem". The same thing can happen if service was done to the hydraulic system along with any other brake work. If the mechanic uses a helper to push the brake pedal when bleeding air out of the system, (which is the most effective method), sometimes that valve starts to move. Ford valves are a REAL misery to center so the light will turn off, but GM and Chrysler valves are spring-loaded. Still, corrosion can build up in the valve body because the valve moves so seldomly. The valve can stick and be so close to turning on the switch that normal braking on your part will be all it takes to trip the light. This can be aggravated by the fact that you have new rear shoes that haven't had time to wear down to where the shoe diameter matches the drum diameter. That's fancy talk for, "the shoes aren't applying equally yet". Also, front disc brakes are automatically adjusted perfectly after the first few pedal applications, but rear shoes will take a long time to self adjust. Your mecanic can only get them close. Over-adjusting in hopes of providing you with a more solid / safe feeling brake pedal can backfire and lead to dragging and overheating rear shoes. It is better to just get the shoes as close as possible, and let the car do the rest over the next few weeks.
I know that's a lot to digest, but it just explains why sometimes these things happen even though nothing was done wrong. Many highly-experienced mechanics figure out ways they can avoid having these things occur. One way is to perform a dozen fairly hard stops during the test drive after the work is done. You would think he was abusing your car, but if done properly, as most service manuals recommend, and allowing the brakes to cool between hard stops, hundreds of miles of break-in can be accomplished in just a few minutes. Had your mechanic done this during the test drive, he MIGHT have seen the warning light turn on, if the pressure-differential switch was the cause. If the problem is just that the new rear linings haven't conformed to the shape of the drums yet, the problem is likely to go away on its own. That is acceptable although most mechanics know it is not an acceptable answer to most customers. He might determine this is the cause and tell you to drive the car for a couple of weeks, then come back if the problem persists. Along with the break-in period, a few good jabs on the brake pedal will free up a valve that's stuck in a little corrosion. At that point, the valve will just sit there waiting for a fluid leak to develop so he can warn you.
Sunday, February 28th, 2010 AT 8:10 AM