[Enoch]

I have fitted the rear brake limiting valve from a pre abs Sierra on to mine. It does not affect the braking under normal conditions but will limit the rear braking effort under heavy braking. You can change the point at which it cuts in by changing the angle that it sits at in the car - it works on inertia. I fitted one to my Robin Hood which passed SVA, the tester ran through the automatic tests and it failed on over braked rear. He knew it had the valve in so took it out in the car park and slammed on the brakes to make sure the rears did not lock. He then passed the car even though the auto test still showed a fail.
If you are fitting a limit valve remember to fit the pressure activated brake light switch between the master cylinder and the valve - if it is installed after the valve your brake lights will turn off when the valve activates. Initially install the valve at the same angle as it was installed in the Sierra - it was about right on my Robin hood so should be about right in the Haynes.
Enoch.