How do Drum Brakes Work?

While disc brakes are the most effective system, the fact is, to reduce cost of manufacture, many cars are equipped with drum brakes on the rear wheels. Note: there are vehicles that come standard with disc brakes on all 4-wheels. To determine your need for Lynnwood Brake Service & Repair, understanding the systems can help you keep your vehicle safe.

Brake Master Cylinder

The elegance of the master cylinder engineering is anything but simple! As one of the most expensive repairs, it is essential that you have your brake system inspected and maintained on a regular basis to avoid master cylinder failure. 

Sequence of Events for 2-circuit System:  

1. Apply pressure to the brake pedal

2. Brake pressure is transferred to the main plunger on the Master Cylinder

3. The rear (blue) plunger moves forward



4. a.) Brake fluid is drawn through 1st circuit intake &return port from the fluid reservoir

     b.) Brake fluid is drawn through the 1st circuit equalization port

5. The 1st circuit seal passes both the intake and return ports

6. Brake fluid level is maintained between the 1st circuit seal and 2nd circuit front seal  for the duration of applied brake pressure

7. Apply more pressure to brake pedal

8. More fluid forced into 2nd brake circuit

9. Fluid pressure builds between plungers

10. Pressure from fluid builds to override the return spring pressure on 1st circuit

11. The 1st circuit seal moves further forward

12. Brake fluid is drawn into the space between the 1st circuit seal and the front of the Master Cylinder

13. Remove foot from brake pedal

14. Fluid is returned to reservoir as the 1st circuit and 2nd circuit seals return to their unpressurized positions

The Brakes

The Master Cylinder is but 1-component of the hydraulic system of drum brakes. Once you press the brake pedal, the process outlined above sends hydraulic (brake) fluid to the Slave Cylinder via hydraulic lines. Often, the Slave Cylinder is actually the Caliper. The sequence of events beyond the Master Cylinder:

1. Pressure from the Master Cylinder is transferred to the caliper

2. The caliper’s piston assembly is activated, applying pressure to both the brake shoe and the brake pads

3. Friction from the brake shoes and brake pads applied to the drum cause wheel movement to stop

4. Depending on the pressure applied, your car will slow or stop


As with any system in your car, the brakes must be inspected on a regular basis. With ‘disposable’ components (brake pads, brake shoes, brake drums), it is expected that they will wear and require replacement from time to time. The brake drums can be machined to achieve tolerance needed for operation only a finite number of times. By inspecting your brake shoes and pads on a regular basis, you can avoid excessive wear on the drum.