what is a proportional valve
 

whats a proportional valve and would it make your brakes spongy?

Re: what is a proportional valve
 

Usually, air in the brake system or a bad front brake line will result in at spongy brake pedal. Could also indicate a leaking or failing caliper, wheel cylinder, master or brake line. Check for leaks around your wheels and below the master cylinder and the rear rubber connector brake line. Inspect your front brake lines and rear connector line for bulges or weeping. Do you need to add fluid constantly?

The purpose of the proportioning valve is to balance front and rear brake bias. The majority of the braking is done by the fronts, followed progressively by the rears. If this part is failing, you should get a brake warning light that constantly displays. Make sure the sensor is hooked up.

If the rears grab and lock before the fronts, you might have a problem with the proportining valve but, I'd look at all of the other and above systems first as this valve shouldnt fail without warning you (above).