Master Cylinders: Notes

Master cylinders have been called "the heart of the hydraulic system." They generate the hydraulic pressure necessary to activate the foundation brake.

The key components of a typical master cylinder are shown in Figure

In its simplest form, the master cylinder consists of a casting with a cylindrical bore containing a piston assembly. At either end of the piston is a rubber sealing surface. The front end is sealed by the primary cup, which is the high pressure seal. The rear end is sealed by the secondary cup. This secondary cup is a low pressure seal which prevents brake fluid leakage from the open end of the master cylinder bore.

The piston is acted upon by a push rod which obtains its force from the effort exerted on the brake pedal. As the piston moves forward, the fluid ahead of it is pressurized. Above the bore is a fluid reservoir. A small hole, directly above the secondary section of the piston called the intake port, allows fluid in the reservoir to enter the bore.

The amount of fluid that moves through the brake system is less than one ounce during a full stroke. The fluid reservoir ensures that the wheel cylinders and caliper bores are filled with fluid as the brake lining and/or pads wear. The reservoir also keeps the secondary area full of fluid during a brake application to prevent a low pressure area.

The master cylinder is fitted with a return spring to help force the piston to return to its rest position when the brake pedal is released. As the piston retracts, it pulls away from the fluid faster than the fluid can make its way back to the master cylinder through the brake lines. In order for the piston to return rapidly and be ready for another stroke, a path is provided to allow fluid to flow to this low pressure area as the piston returns. This path is a series of small holes located near the outer edge of the master cylinder piston. These holes allow fluid to flow from the secondary area behind the main cup around the edges of the cup and into the primary area. The reservoir supplies brake fluid through this intake port as necessary to keep the secondary area full during the complete piston stroke.