https://mattersoftesting.blog.gov.uk/changes-made-to-brake-performance-test/

Changes made to brake performance test

On 23 June 2013 changes to the brake performance test procedures and standards were introduced. The changes are not complicated, but it is important that they are well understood.

The minimum brake efficiency requirement for ‘M1’ vehicles first used on or after 1 September 2010 has been increased to 58%.  ‘M1’ vehicles are defined as: passenger vehicles with 4 or more wheels and not more than 8 passenger seats in addition to the driver’s seat. Obviously this covers cars, but also includes ambulances, dual purpose vehicles, motor caravans etc. Quadricycles are not included as these are in category ‘L’.

Brake imbalance is now only measured at maximum brake efforts, regardless of wheel lock. The failure criteria has been relaxed to greater than 30% rather than the previous 25%.  Additionally, with the exception of trikes and quads, imbalance must be checked across all axles.

These changes have resulted in a simplified manual roller brake test procedure, which no longer requires checks for brake grab or judder, but includes a new check for fluctuation of brake effort.  The brake data entry screens on MOT Computerisation will also look slightly different, but the VTS device will automatically apply the appropriate criteria when calculating brake efficiency, and will also calculate brake imbalance.
  
Those few vehicles still around with a single line braking system will be subject to some additional checks. This is because the parking brake on such vehicles also performs the secondary brake function and will therefore be checked for severe ‘grabbing’ during application as well as for imbalance across the axle. As for the service brake, there must not be more than 30% imbalance measured at maximum efforts.
 
Testers should familiarise themselves with these changes by reference to the standards and procedures detailed in Inspection Manual Sections 3.7, 3.8 and 3.10.