The proportioning valve balances braking force between the front and rear of your car. When it fails, the balance goes wrong - and you feel it as rear brakes that lock too easily or barely work. Here is how a proportioning valve fails and what it feels like.
What the valve does
The proportioning valve limits pressure to the rear brakes so they do not lock before the fronts - important because the front brakes do most of the work and the rear has less weight over it. It keeps braking balanced and stable, especially under hard stops.
Symptom: rear brakes lock early
If the valve fails in a way that sends too much pressure rearward, the rear brakes can lock up prematurely under braking, making the back end feel loose or causing a skid - see why brakes lock up. This is unsettling and unsafe, especially on wet roads.
Symptom: rear brakes under-apply
The opposite failure starves the rear brakes of pressure, so the fronts do nearly all the work - the fronts wear fast and overheat while the rears barely wear. Combined with a low or long pedal, a valve issue can also lengthen stops.
Diagnosis and fix
A failing proportioning valve is diagnosed by comparing front and rear braking behavior and pressures; on many cars it is part of a combination valve with the brake warning switch. It is not a DIY-guess part - have it tested if the front/rear balance seems wrong. Meanwhile keep pads, rotors, and fluid healthy with quality pads and hardware.