Bleeding a master cylinder on a car is the process of removing air from the hydraulic system that actuates the brakes. Air in the system can cause a spongy brake pedal and reduced braking performance. Bleeding the master cylinder is a relatively simple procedure that can be performed with a few basic tools.
The master cylinder is located in the engine compartment and is connected to the brake pedal. When the brake pedal is depressed, the master cylinder pushes brake fluid through the brake lines to the calipers or wheel cylinders. The brake fluid creates hydraulic pressure that forces the calipers or wheel cylinders to squeeze the brake pads against the rotors or drums, which slows or stops the car.