The most likely cause of a brake pedal falling to the floor while the vehicle is stopped is a leaking master cylinder primary seal. This allows high-pressure fluid to bypass into the low-pressure chamber.
Brake systems are either longitudinally (front/rear) or diagonally split, providing safety in case of a failure. A master cylinder contains primary and secondary pistons, each with seals. The primary seal on both pistons holds pressure in the high-pressure chambers. A leak in a primary seal causes the pedal to drop.
Master cylinders also feature a low sensor float that magnetically triggers a warning light when fluid levels are low.
When opening a brake system, air can enter, so bench bleeding the master cylinder is recommended. This involves filling it with clean brake fluid, stroking the piston to remove air bubbles, and then capping it before installation. While bench bleeding removes most air, further brake bleeding is always necessary.
Worn front pads or misadjusted rear shoes can cause a low pedal, but not specifically when the car is stopped. Air in the system results in a spongy pedal. Understanding these components is crucial for the A-5 AE brake exam.
Master Cylinder Bench Bleeding
