How Drum Brakes Work | Lynnwood Ford Brake Repair & Service

While disc brakes are more efficient and effective, some cars use drum brakes in combination for economic reasons. Generally, if disc and drum brakes are used in concert, you will find disc brakes on the front and drum brakes on the rear of the vehicle. To learn more about disc brakes, see previous post: How Disc Brakes Work. Should you be in need Lynnwood Ford Brake Repair Service, Carson Cars Brake Repair Mechanics offer certified technicians to diagnose and service/repair any brake system!

Brake Actuator System

More commonly referred to as the hydraulics that transfer the force applied by your foot to the brakes, the actuator system employs basic physics to stop a vehicle. A typical single-circuit hydraulic brake system has the following components:


  • Master Cylinder
  • Slave Cylinder
  • Reservoir
  • Hydraulic Hoses filled with non-compressible hydraulic fluid (brake fluid)

When you push the brake pedal, a small piston assembly is compressed in the master cylinder, which sends the resulting pressure through the brake fluid’s hydraulic lines to the slave cylinder instantly. Generally, that slave cylinder is in the form of a caliper; the caliper activates another piston assembly — pushing it out — acting directly on the brake pads on the brake shoe, which use friction (acting in the opposite direction of wheel movement) to stop your Ford.

The Master Cylinder and Power Assist

Braking power is enhanced with assistance from the engine or the battery. The most common power brakes:

  1. Vacuum Suspended
  2. Air Suspended
  3. Hydraulic Booster
  4. Electrohydraulic Booster

The complicated engineering of a master cylinder is genius! In this simplified diagram, you can see the basic operation of this component. Stepping on the brake activates the main plunger that acts on the internal components of the cylinder. As the rear (blue) plunger moves forward, brake fluid is drawn from the reservoir (on top) through the 1st circuit intake and return port; simultaneously fluid is drawn in through the equalization port. Once this seal passes the intake and return port, a fixed volume of brake fluid is maintained between the 2-plungers. As more pressure is applied to the brake pedal, more brake fluid is forced into the 2nd brake circuit, operating the brakes. As the pressure builds between the 2-plungers, the 1st circuit return spring pressure is over-ridden and the 1st circuit (red) plunger begins to move forward and draw fluid in, filling the space between the front seal and the front of the master cylinder. Note that the 1st circuit has 2-seals — fluid is drawn in until front seal passes the fluid intake and return port. The pressure behind the fluid forces it to the 1st brake circuit — applying those brakes. Removing your foot from the brake pedal, returns the master cylinder system to an unpressurized state. In this neutral state, the hydraulic fluid is drawn into and stored in the brake fluid reservoir.

As expected, the master cylinder is one of the most expensive items to replace in the brake system. With regular inspection and service of your drum brakes, you can avoid catastrophic failure. The experts at Lynnwood Ford Brake Service can inspect your brake system with each oil change.