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What Master Cylinder Do I Need?

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Your master cylinder is the heart of your braking system. In order for your system to function properly you must utilize the correct cylinder for your system's demands. The following brief descriptions should help you in determining what master you will need.
DRUM / DRUM A drum / drum master is designed to deliver fluid pressure and volume to the front and the rear of a braking system in equal proportions. Drum brakes will require less fluid and pressure than disc brakes. Typically a drum brake master will be smaller than a disc master and the fluid reservoir chambers will be equal in size. Since drum brakes require the use of residual pressure valves the original drum master cylinders had residual valves built into the outlets. Later model aftermarket units do not have these valves and they must be installed in the lines externally. Failure to incorporate residual valves will cause spongy brakes.

Generally speaking it's not a good idea to use a drum brake master for disc brakes since the amount of fluid the cylinder pushes will not be sufficient.
DISC / DRUM A disc / drum master is designed to push more fluid volume to the front disc brakes since disc brakes require more volume than drum brakes. Generally speaking a disc / drum master will have one fluid reservoir larger than the other.This is because the disc brake pads wear faster than the drum shoes and the fluid reservoir will drop faster. Original disc / drum masters had a built in residual pressure valve to the rear drum brakes only.

Never reverse the outlets on a disc / drum master. Doing this will cause the front disc brakes to drag excessively from the residual pressure valve.
A four wheel disc brake master cylinder is designed to supply more fluid pressure and volume to the rear disc brakes than the disc / drum master does. This is acheived through an internal piston re design. The piston that feeds the rear brakes on a disc / drum master will run out of stroke, limiting the amount of fluid pressure and volume that may be supplied to the rear isc brakes.

The four wheel disc master re design delivers the extra needed volume and pressure to the rear allowing your rear disc brakes to function properly. If you attempt to use a disc/drum master on a four wheel disc system you will get poor rear brake function and experience a spongy brake pedal with a long pedal travel.