"The bias valve fitted to twin circuit braked Minis is to ensure that the line pressure to the rear brakes of the car is limited when the braking effort is above a predetermined level. This is to ensure that the front brakes will always lock before the rear brakes and ensure stability during heavy braking.
As weight is transferred to the front axle under braking, the contact pressure between the rear tyres and the road will become less than that at the front wheels, and therefore the braking torque required at the rear to lock the wheel is less.
If the rear wheels locked before the front then this is inherently unstable (the car could spin out of control), whereas locked front wheels will cause the car to skid straight ahead.
Earlier minis, with single line brakes, used a pressure limiting valve fitted to the rear subframe. The later valve allows the rear brake pressure to rise only up to a certain value, over which the pressure to the rear remains constant whilst the front pressure increases with further pedal effort.
You can safely ignore the PDWA switch and use a low fluid indicator as fitted to later Minis, but I would leave the old switch in to ensure that the valve is sealed adequately. Do not be tempted to remove the valve however!"