Most classic Minis will now have the later ring type main seal on the upper end of the internal piston in both clutch and brake master cylinders. The Mini has always used a .75" bore clutch master cylinder, and some early Minis also used a .75" brake master cylinder. This repair kit will fit either type, the plastic valve is not used in clutch applications and can be discarded.
Depending on the age of the master cylinder you are repairing, it could have either the modern ring seal on the end of the internal piston, or it could have a cup seal if the master cylinder is a very old unit. If you are uncertain, it is normally best to dismantle the cylinder and determine which type of seal is used. The early piston has a flat upper end and a cup seal, the later type has a pointed upper end with a ring seal. Externally, some master cylinders have a small groove machined on the outside of the neck of the cylinder, near the bottom.
Early master cylinders with cup main piston seal require kit GRK3007. Use GRK3007-KIT to update it to the later, improved version