Does anyone know if abs pumps have a rear brake pressure limiting valve in them?. My 4x4 has always just scraped though the MOT due to low service brake efficiency, the fronts lock on the rollers without a problem but the rears only get to 200kgs. Prefect balance but only 200kgs I found out that the master cylinder has a valve in it that only allows pressure to the rear brakes after a certain pressure is applied to the fronts so I thought it's got to be that. Fitted a new master cylinder and it's exactly the same. I've solved the problem by fitting some 43mm rear calipers in place of the original 40mm ones the pedal travel is a little longer but still within limits and the effort is up to 250kgs which is an easy MOT pass. But I would love to know what's causing the problem.