The yellow ABS warning light will not cause the red Brake light to turn on, but the red Brake light will turn the yellow ABS light on. That's because the ABS Computer only knows something is wrong with the base brake system but it doesn't know what. That problem could result in an inability to run the anti-lock function properly.
That means you need to look at the red warning light, and there's only three switches at most that affect it. The first, if your truck has it, is a low-fluid level switch in the master cylinder. If the fluid is low, do not fill it. Add just enough to turn the light off. The fluid is low either because there's an external leak that must be addressed or the front disc brake pads are worn and need to be replaced. Replacing worn brake pads requires pressing the pistons back into the calipers, and that pushes all the brake fluid behind them back up into the reservoir. If a do-it-yourselfer or inexperienced mechanic has previously filled the brake fluid, it's going to overflow and run all over creating a mess, and damaging any painted surfaces.
The second switch is for the parking brake. Be sure the pedal is releasing fully. The last switch is the pressure-differential switch. That may be in the hydraulic controller for the ABS system, otherwise it is in the middle of the brass combination valve that sits on the frame rail right under the master cylinder. On vehicles without anti-lock brakes, you can follow the two steel lines from the master cylinder to that valve. Simply unplug that switch to see if the red warning light goes off. If it does, one of the two hydraulic systems didn't build pressure as fast as the other one. Typically that's due to a leak. That valve is spring-loaded on GM and Chrysler vehicles but sometimes it sticks. A quick hard jab on the brake pedal usually releases it.
Saturday, May 2nd, 2015 AT 8:55 PM