schematics and diagrams: The Brake Indicator Light Stays On dash of car?

As you know, the Brake light on your instrument panel tells you when your parking brake is set, right? But, in most cars, this light is also used to indicate a fault in the brake system. In the case of a fault, the light will stay on even when the parking brake is released. I will go through several common reasons for the light to stay on and some troubleshooting procedures to help you resolve the problem.

1. Parking brake switch broken or misaligned.
Your parking brake has a switch, usually at the base of the handle, that turns on the light when you set the brake. The common design is a button that closes (pushes in) when the brake is completely released. If you can find this switch, try pulling the connector from it. If the connector has only one terminal, ground it to the body of the car to see if that will put the light out. If it has 2 terminals, connect them using a paper clip.
If grounding or shorting the connector puts out the light, your switch is bad--replace it.
If this doesn't put the light out, it is possible that there is an open in the wiring. If you can trace the wire and short it at the other end, you can further eliminate the possibility that this is your problem.

2. Low brake fluid level. Check the brake fluid reservoir for adequate fluid level. If the level is low, add brake fluid, and inspect the car for leakage. If the level is only a little bit low, it may be due to normal wear of the disk brake pads.
But if you have to frequently add brake fluid, you have a leak. Places to look for common leaks are as follows:
All four wheels under the hubs.
Under the master cylinder in front of the driver inside the engine compartment.
At the clutch slave cylinder if you have a manual transmission without a separate clutch reservoir. The picture below shows a brake fluid reservoir and a hydraulic clutch reservoir side by side:

3. Leaks. If you have a leak at one or more slave cylinders or calipers, the slave must be replaced and the system bled of all air. Leaks may also occur at junction blocks or at the ABS module, or even in a brake line that has corroded through. Due to the wide variety of brake systems and configurations, I will not cover repair procedures but will provide a few tips.
First make sure you identify the exact location of the leak. For example, on a disk caliper, if the leak is at the bleeder or line connection, you may be able to stop it by tightening the fitting. If the leak is coming from the piston, replace the caliper.
With drum brakes, a slave leak can be verified by pulling open the rubber boot on each side on the cylinder. If you get a good drop or 2 there, replace the brake cylinder.
Note: A key tip to limiting your bleeding effort is to have the new part ready to install before disconnecting the brake line. Then quickly install the new part before losing fluid from the brake line.
Bleeding: Once the system has been opened by replacing a slave or caliper or if air entered the system due to an empty reservoir, air must be bled from the system.

Always use a catch pan when bleeding, and a hose on the bleeder if you prefer, to direct the fluid into the pan. Clean up any spilled fluid, as brake fluid is corrosive.

If the problem was at a particular wheel, you can probably just bleed the one wheel. But if the reservoir ran dry, you will have to do a full bleed.
A full bleed is alway done wheel by wheel beginning with the wheel furthest from the master and proceeding to the wheel closest to the master.

Some cars will gravity bleed by just opening the bleeder valve at the top of the slave (and keeping the reservoir full) until all bubbles have passed and you have a purely liquid flow. If the line will not gravity bleed, have a second person pump the brake pedal and hold it down while you open and close the bleeder valve. Do this repeatedly until no air comes out of the bleeder.
If your car is equipped with a proportioning valve, a better procedure is to start with the pedal up, open the bleeder, push the pedal down, close the bleeder, allow the pedal to retract, wait 5 seconds, repeat.

If your pedal is still soft after bleeding, it is possible that you have a bubble in the ABS system. Check with your dealer or repair manual on how to bleed the particular ABS system or have the system bled professionally.

If after repairing a leak and bleeding the system your brake indicator does not extinguish, you may have a stuck proportioning valve. Many designs employ a proportioning valve to prevent the back wheels from locking up. The valve is often integral to a tandem master cylinder (but can be separate) and can incorporate a switch that will light your brake indicator when there is a leak in the system. If the leak is repaired and the system is bled properly, the switch should reset on its own. Try removing the connector at the proportioning valve to see if that will extinguish the light. If it does, the switch has failed closed due either to a broken valve spring or a short. Replace the valve and bleed the system.

4. Sinking pedal, no leaks. If your car has no brake fluid leaks but the pedal sinks under constant pressure, your master cylinder has failed and must be replaced. Make sure to bench bleed the new master cylinder and plug the ports before installing. Also be carefull to limit the fluid dripping from the lines as the old master is removed. If there was no air introduced to the system, you may not need to bleed the system after replacing the master cylinder. If the pedal is soft, try just bleeding at the master cylinder line nuts. If this doesn't work, you'll need to do a full bleed.

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