|Updated 04/13/04 Submitted by: John Paseman
Whether you need a proportioning valve will depend on many factors - brake size front & rear, weight balance front / rear, how much weight transfer (nose dive) your car exhibits under hard braking are just a few.
If you can press the brakes hard and the rears do not lock up - you will not need a proportioning valve. If they do (careful when you try this - it can be dangerous!) then you will need to add a proportioning valve in the rear brake line. The factory engineers would experiment to find a good compromise valve setting and then mass-produce when needed. For you, an adjustable version makes the most sense.
You start with the valve open and keeping closing the vale and testing for rear brake lockup until they no longer lock-up under hard braking. Find a nice deserted parking lot and go lock-up the brakes.No need to slide the tires a lot as it will trend to wear tread kinda fast.
The theory on this is that the front brakes do most of the stopping. If the rears lock before the fronts, those tires will do NOTHING for braking (while they are sliding once they lock up). The factory valve delayed the onset of the rears just a bit so they would not lock up before the fronts started to slow the car down. An adjustable valve lets you fine tune your brake system to your exact car (and needs).