Efficiency of the brakes:

Brake performance is tested using a roller or plate type machine. Certain vehicles must be tested dynamically on the road i.e limited slip diffs, permanent 4 by 4's, transmission driven servos and those vehicles where the weight cannot be determined i.e motor caravans. Limited slip differentials can be identified when jacked i.e if both wheels turn in the same direction when one is rotated, its limited slip.

If the vehicle has defects which might cause a problem if a roller brake test was carried out then that part of the test will not be completed. Similarly if a road brake test were inappropriate the same action would occur. A full brake test would have to be carried out at re-test.

The majority of cars will need to achieve 50% efficiency on the foot operated system.  If the system is a single line hydraulic arrangement the parking brake must get 25%, otherwise 16%.

Efficiency is calculated using the vehicle weight from a chart, this weight includes an element for fuel, passengers etc. The formula is:

Brake force/Vehicle weight *100

If more than half the wheels "lock up" then the vehicle has met the requirements irrespective of any calculated performance. Of course, any wheel which is not functioning properly is likely to fail under other sections of the test.

Out of balance in excess of 25% across the steered axle will fail. Out of balance is not a testable feature on the rear axles.

On both axles the brakes must come on and off more or less together, the purpose of this is to identify sticking wheel cylinders or lazy disc calipers.