ABS and brake light on

The red "brake" warning light turns on for one of three reasons. The fluid level is low in the reservoir, the parking brake pedal is not fully releasing, or unequal pressures are building up in the two halves of the hydraulic system, typically due to a leak, and the "pressure-differential valve" has turned the light on.

The ABS computer sees the red warning light is on but it does not know why. It only knows that with low fluid, a leak, or a dragging brake, it likely will not be able to pulse the brakes properly, so it turns the system off and turns the yellow "ABS" light on to tell you. Most of the time, fix the red light and the yellow one will go off too. If there is a problem detected within the ABS system, that will turn on the yellow warning light but not the red one.

If the fluid level is low, there is either a leak that must be addressed, or the disc brake pads are worn to the end of their life. As they wear, the pistons move out of the calipers to self-adjust. Brake fluid fills in behind them. The low fluid level in the reservoir is the clue, not the symptom. We never top off brake fluid during other routine services such as oil changes, because when the new brake pads are installed the pistons have to be pushed back into the calipers. That pushes the brake fluid back up into the reservoir. When do-it-yourselfers added fluid previously, pushing the pistons back forces the fluid to overflow and gush out of the reservoir, creating a huge mess. Brake fluid eats paint too.