The proportioning valve balances braking between the front and rear axles. When it fails, the rear brakes do too much or too little. Here are the symptoms of a bad proportioning valve.
Rear brakes lock too early
If the valve fails open, the rears get full pressure and can lock up under moderate braking - especially on a slippery road, causing the back end to slide; see brake proportioning valve. Early rear lockup is a classic sign.
Weak rear braking
If the valve sticks closed or over-restricts, the rears barely engage, so the fronts do almost all the work and wear fast; see how it fails. You may notice long stops and front pads wearing out quickly.
Uneven pedal or pull
A malfunctioning valve can make the pedal feel inconsistent or cause a slight pull as braking balance shifts. On vehicles where it is part of a combination valve, the brake warning light may also come on.
What to do
Have the front-rear balance checked; a stuck valve usually needs replacing. Rule out other causes like a sticking caliper first; see brake diagnosis. Keep the whole system healthy with quality pads and hardware.