Hi ahnom; Randy here. Here's a copy / paste of a reply to a similar question from someone who wanted to eliminate the valve. Just pull out the information you can use:
No! Don't eliminate it. That's a height-sensing proportioning valve. These are rarely used on cars but are common on higher-quality trucks and minivans.
Cars have some pretty sophisticated valving to prevent too much hydraulic brake fluid pressure from going to the rear brakes. As you step harder and harder on the brake pedal, most of the weight of the vehicle transfers to the front. As the rear of the car goes up, the rear wheels would lock up if fluid pressure wasn't controlled. A skidding tire has very little traction and increases stopping distance.
Manufacturers can't design a one-size-fits-all proportioning valve for minivans and trucks because there is such a great difference in the amount of weight that could be in the back. Valving designed around an empty vehicle could eliminate rear wheel lockup but would leave the braking system under-powered when trying to stop quickly with a heavy load. Valving for a fully-loaded vehicle would result in very easy, dangerous rear wheel lockup when lightly loaded. The adjustable valve maintains the strongest possible braking system under any weight distribution condition.
In addition to this valve, manufacturers do a lot of research in the friction coefficient of the brake linings. That means how much friction is developed for braking. It used to be common to find cheaper-quality pads and shoes that had a higher coefficient of friction. That means they would need less pedal pressure to stop just as hard. Sounds good at first until you realize that the designed-in balance is gone and the rear wheels might lock up easily or not have full stopping power.
On a related note, with age, easy rear wheel lockup becomes somewhat more common, especially on wet roads. This can be caused on any vehicle with rear drum brakes if the parking brake cables are sticking and not fully releasing. This is REAL common on Ford products, but can happen on any car or truck. On most cars and trucks, you can tug on the cables and watch if they release by themselves.
If the cables are free, but easy rear lockup occurs, the valves on Caravans are adjustable. The link between the leaf spring and valve is a two-piece link with a small bolt and a 5/16" or 8mm nut. Loosen the nut a little and let spring tension stretch the link about 1/16". You can determine the amount of change by observing the shiny circular scratch in the paint made by the nut. Don't stretch the link more than that. If easy rear wheel lockup still occurs, look for grooves in the rear brake backing plates, manually over-adjusted shoes, or the rear shoes installed backwards. (The longer lining goes on toward the rear of the vehicle; the shorter lining goes to the front.
Besides being a necessary part of the braking system, this valve provides a convenient place to install a new steel brake line without having to go all the way from the master cylinder to the rear brake. There's two lines going in and two going out to the brakes because this is a "split-diagonal" system. The left front and right rear are on the same hydraulic circuit. This is done to insure one front brake still works if there's a leak in the other circuit.
This valve rarely fails. If yours is damaged or perhaps leaking, (although I've never seen that), I would not be worried about finding a good used one in a junkyard. Since I've never replaced one, I'd have to send you to the service manual for the adjustment procedure rather than relying on trial and error.
Saturday, March 28th, 2009 AT 7:32 PM