# What Is a Brake Slave Cylinder?

January 27, 2012

The braking system in your car is a hydraulic system that uses a master cylinder and brake slave cylinder to generate fluid pressure in response to brake pedal movement. Given that information on a brake slave cylinder can be tough to find, one might think that it is some new invention. The truth however, is that slave cylinders simply go by a different common name. This can lead to much confusion about what they are and what they do.

#### How Brakes Work

With the help of a brake booster, a device that uses a vacuum to multiply the amount of force used to actuate the master cylinder's piston, fluid is pushed through the brake system when you press down the brake pedal. This pressure makes its way through your brake system, eventually acting on the individual slave cylinders located at each of the car's four wheels. This pressure pushes pistons down their bores in each slave cylinder, which in turn pushes on the brake pads and stops your car.

#### Leverage Is Key

In a transmission, torque is multiplied by using larger diameter gears which in essence increase or divide the amount of force transmitted across them. If you exert one pound of force on a 1-inch diameter gear which in turn exerts that force on a 2-inch diameter gear, the amount of force exerted by the 2 inch gear will be 2 pounds. A slave cylinder works in a similar fashion, but with fluid pressure. A master cylinder displaces a given amount of pressure as its piston moves down its bore, and this pressure is based on the diameter of the cylinder's piston. This pressure can then be further augmented by changing the number and size of the slave cylinders the pressure is acting on. Smaller cylinders will deliver less pressure as their pistons are displaced, while larger cylinders will deliver more. This ability to alter fluid pressure is essential to properly distributing braking force from wheel to wheel.

Slave Cylinders on a Modern Braking System

To find exactly where your slave cylinders are located on your car's braking system, look no further than your brake calipers. Inside your brake caliper are a number of pistons which directly press on the brake pads. The number and size of these pistons are generally bigger on the front of the car, because this is where the bulk of a vehicle's weight transfers during braking. Calipers in the rear of the car are generally smaller and fewer in number because there is less braking work to be done in the rear, and therefore less braking force is required. When the brake pedal is pressed down, a proportioning valve divides fluid pressure between the front and rear, depending on a car's braking needs. From here, it is up to the individual slave cylinders to translate this pressure into braking force.

A brake slave cylinder is simply the proper term for each piston and bore found in each of your brake calipers. They are called slave cylinders because they only work when forced to by the pressure exerted by the master cylinder, so this naming system is appropriate for any hydraulic system.