What is wrong when the brake pedal goes to the floor?

A defective master cylinder and low brake fluid are common reasons why a brake pedal goes to the floor. The brake system is pressurized by the master cylinder when the pedal is pressed. It provides hydraulic fluid to the wheels. A red brake-warning light typically illuminates with hydraulic failure.

Low brake fluid may indicate a leak in the master cylinder. Leaks occur in flexible hoses and hydraulic lines. Worn brake pads can also cause low fluid levels. Calipers and brake shoes should be checked for fluid contamination. It is often necessary to replace the master cylinder and bleed the hydraulic system of air.