A Master Cylinder Vs. a Slave Cylinder


The master and slave cylinder relationship is about what you'd expect from the names; a master tells the slave to push or pull out, and the slave does what it's told. All hydraulic systems work on this same principle, varying only by degrees of systemic complexity.

Hydraulic System Basics

  • An hydraulic system is essentially a mechanical linkage just like a throttle cable or shifter linkage. The hydraulic fluid in the system is incompressible, meaning that no matter how hard you push the molecules that make up the fluid they will never get closer together. Applying force to one end of a sealed hydraulic system will force the fluid to press against whatever is on the other end of that system. Simply put, the syringe-like master cylinder pushes fluid through the lines, and the slave cylinder receives the fluid and moves in accordance.

How a Braking System Works

  • When you press on the brake pedal, it rotates on a pivot to push a rod. That rod connects to the master cylinder, which often receives some assistance from a booster connected to a vacuum source on the engine. Fluid displaced from the master cylinder goes through a metering block, which directs fluid pressure to the appropriate wheel. An antilock braking system uses a number of servos to control the sizes of the orifices in the metering block, thus increasing or decreasing braking force to the individual wheels as circumstances dictate. Finally, the slave cylinders in the brake calipers receive the fluid pressure to clamp the brake pads' friction material against the spinning brake rotors or drums.

Master vs. Slave Cylinder Design

  • The plunger inside of a master cylinder may travel as much as 2 inches from the fully open to fully closed position, but the slave cylinders in the brake calipers need only travel about 1/8 inch to apply the brakes. Assuming the brake fluid gets distributed evenly to all four wheels --- which it doesn't, since the front wheels do most of the stopping --- the slave cylinders must be about three to four times larger in diameter than the master to receive all of its fluid without overextending. This difference in cylinder diameter results in a sort of "gear reduction" between the master and slave cylinders, allowing the slave cylinders to exert more pressure on the rotors to increase braking force.

Caliper Design

  • Most brake calipers are sliding or "floating" designs that use a slave cylinder on one side only. A set of steel sleeves encircle the caliper bolts, filling the space between them and the brake caliper bracket. As the slave cylinder applies pressure against the rotor, the entire caliper slides inward to pull the other brake pad against the rotor. The inner surfaces of these sleeves need to be greased during every brake pad replacement or the caliper may seize up and refuse to slide when the slave cylinder applies pressure.

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