Adjustable original type brake proportioning valve.
The most common brake system on the Mini has Disc brakes in the front and drums in the rear. The weight bias on the car is all in the front & thus under hard braking all of the weight is transferred to the front. When this happens it is very easy to have the rear brakes lock up before the front.
The standard Mini both 4 wheel drum and later model disc brake cars has a special valve in the hydraulic line from the front to the year. This valve 'limits' the amount of fluid pressure that can be applied to the rear. The valve is the same for all models that used a single cylinder master.
Most of us have over the years changed the car in some fashion, bigger wheels, different ride heights, etc. that would upset the front to rear balance. If you want to get this balance back again the only real answer is the adjustable proportioning valve. The simple adjust is by a "T" handle on the back of the valve.
Must be set up professionally. Many race and rally car owners fit the brake lines and valve inside where possible for safety and easy adjustment.