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Article: Master Cylinder Identification Guidelines

Posted on 1st December 2011

This original article was previously published by Seven Enterprises in our Autumn 2011 mailer.

Master Cylinder Identification Guidelines

by Mike Kearney
Seven’s Service Shop Manager

Identification of the master cylinders is a common problem that Mini owners encounter when working on an older car or when building a “basket case” from parts of unknown origin. It really is not too difficult; here is a list of facts to check to minimize the guesswork.

Visual Identification
If you have a basket case car and need to identify which cylinder is the clutch, hold the master cylinder vertically so the filler cap is towards you and the outlet neck is away from you. If it is a clutch master, the pointy end of the mounting flange will be aiming back at you. The centerline of the two mounting bolt holes will be roughly, but not exactly, parallel with the centerline of an imaginary line drawn through the centers of the filler hole and outlet neck. Any master cylinder not passing this test will be a brake master cylinder. One other easy identification is that if you have the large reservoir Cooper S brake master cylinder, the tank will measure 3” exactly from top to bottom of the reservoir. Heritage has recently reproduced the large reservoir Cooper S brake master cylinder in a gold/cad finish vs. the original grey.

Clutch Master Cylinders
All Minis have 0.75” bore clutch masters. See “identification” above.
New clutch master cylinders are available in the original metal tank design as well as two types of improved plastic tank versions.

Single Line Brake Master Cylinders
Minis with single circuit brake systems originally came in three bore diameters: 0.625”, 0.70” and 0.75”. Bear in mind, the master cylinder should match the type of brakes now on the car for safety and reliable performance as follows:

  • The original application of the 0.625” bore cylinder was in the 998cc Cooper only, but is no longer available, so 0.70” should be substituted.
  • The original application of the 0.75” bore master cylinder was the very early single-leading-shoe drum brake cars. Some people now use this diameter master cylinder for 4-piston caliper conversions where some extra fluid volume is required. Nearly all other cars used 0.70” diameter, which is by far the most common. The original master cylinders had metal reservoirs, affectionately known as “bean cans”.

Dual Line Brake Master Cylinders
These can be grouped into three categories, “Round Reservoir”, “Square Reservoir”, or “Horizontal”. These feature separate brake line systems so that if one cylinder or caliper fails you still have partial braking.
- Round reservoir brake master cylinders can be found on Canadian-made Minis, Innocenti, other European Minis and even the Austin America. Some of these cars used a diagonal split of the two brake systems, others were split front to rear. This cylinder is now obsolete, and rare old stock sells for very high prices. Rebuild kits are available if you have a servicable old unit.
- The rectangular reservoir came next, with various versions manufactured up to 1989. Only the GMC0227 replacement version is available today.
- From 1989-on a horizontal master cylinder was used, in conjunction with a power brake servo.

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