Identifying your Master Cylinders

Minis and Mokes have been fitted with a variety of different brake and clutch master cylinders over the years of their production and it is important to know what type of master cylinder your car is fitted with, as the rebuild kits vary for each type. The original equipment master cylinders can be broken down into three main categories - early tin tank style, later plastic tank style and finally the twin-circuit style.

Tin Tank Style

Early Minis were fitted with tin tank master cylinders for both brake and clutch. These were generally fitted to all Minis until 1969. There were different variations of tin tank master cylinders, which are identified by whether they have a groove around the base of the master cylinder. The type without the groove ('no groove' type) has different seals internally - one o-ring style seal and a cup seal. The groove type has two o-ring style seals. Therefore it is important to identify your master cylinder before purchasing a rebuild kit so that the seals are correct.

The rules for groove versus no-groove master cylinders is applicable for both brake and clutch tin tank master cylinders. Do note however that the internal bore size differs for brake and clutch - tin tank brake master cylinders are 0.7" bore diameter and clutch is 0.75". It is possible to swap a brake and clutch master cylinder as they are identical internally other than a one-way valve fitted to the brake master cylinder. Therefore it is recommended to measure the bore size of your master cylinder. Sometimes the bore size is stamped on the cup seal on no-groove type master cylinders.

There are different variations of tin tank master cylinders, identified by a groove around the circumference of the base of the master cylinder

Plastic Tank Style

Plastic tank master cylinder as fitted to Australian Minis from 1969 to 1976

Australian Minis from around 1969 to 1976 were fitted with plastic tank master cylinders, made by either Lockheed or Girlock. Lockheed master cylinders have a cast iron body and the Girlock master cylinders have an alloy body, but are otherwise the same. In contrast to the tin tank master cylinders, plastic tank master cylinders are 0.75" bore diameter for the brake, and 0.7" for the clutch.

This style of master cylinder is no longer available new.

Twin Circuit Style

Late model Minis and Mokes from around 1976 were fitted with a twin circuit braking system (sometimes referred to as a tandem system) for added safety. The style of master cylinder fitted to Australian cars has a rectangular reservoir, which is different to most English cars which had a circular reservoir. The style used on Australian cars is also found on some early Japanese imports and some English cars.

Minis and Mokes which were fitted with twin circuit brakes also featured a 'Brake Fail' light to warn you of impending doom. This warning light was activated if there was a significant difference in pressure between the front and rear brake circuits, which would suggest failure in that circuit. Early cars had this sensor in the pressure limiting valve directly below the master cylinder, later master cylinders having it inbuilt. The Australian style of warning light switch, with a triangular two pin plug, is no longer available, however a English type switch can be fitted in its place.

Cars fitted with twin circuit braking system used a tin tank master cylinder for the clutch.

Australian style twin circuit master cylinder

Brake limiting valve fitted below master cylinder