Blog: Tips for the GMC159/GMC160 Master Cylinder

by Seven Mini Parts | Thursday, February 7, 2013

A brief discussion of the differences between various master cylinders

BMC Special Tuning offered under part number C-AJJ3388 a tandem brake master cylinder conversion kit using the GMC159 fore/aft split cylinder. Special Tuning closed 30 years ago, so do not expect to find the kit sitting on someone's shelf.

The GMC159 master cylinder was originally used on 1968-75 Canadian model Minis, 1969-1971 Innocentis, and later Austin Americas. From approximately 1976-1977 some Minis had a diagonal split system, and used GMC160. The same main casting was used on the GMC159 and GMC160 diagonal split master cylinder, so, externally, one can't tell the two versions apart. There are slight internal differences, such as one piston is slightly different, so these are not interchangeable with the GMC159, which is the one you should find for this conversion. You also will need a longer pushrod from the pedal, the one in the kit was 37H2769, which was 3.5" from end to end. You plumb the bottom outlet of this cylinder to the front and the top to the rear. These master cylinders are no longer available new, but rebuild kits are available if you have a used one.

One tip that ST spelled out is that once this tandem master is installed, one needs to bleed BOTH front and rear cylinders AT THE SAME TIME. These bold instructions are on the original sheet, and read: "Three operators and two sets of bleeder equipment will be required."