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 and thus under hard braking ALL of the weight is transfered to the front. When this happens it is very easy to have the rear brakes lock up before the front. Not only can this be un-nerving; it can be down right unsafe! The stock Mini both 4 wheel drum and later model disc brake cars has a special valve in the hydraulic line from the front to the rear. 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.