If the front-to-rear brake force isn't balanced correctly by the proportioning valve, the rear brakes will receive too much brake force causing them to lock up and skid when the brakes are applied.
The other reason for using a proportioning valve to reduce hydraulic pressure to the rear brakes has to do with the design of the brakes themselves. When hydraulic pressure is applied to the wheel cylinder inside a drum brake, the shoes are pushed outward against the drum. When the shoes make contact, the rotation of the drum tries to drag them along. But since the shoes are anchored in place, the drum only pulls the shoes up tighter against itself. Because of this, drum brakes that are "self-energizing" require little additional pedal effort once applied.
Disc brakes, on the other hand, are not self-energizing. It takes increased pedal effort to squeeze the pads against the rotor. In a vehicle that has front disc brakes, and drums in the rear, the drums generate increasingly greater amounts of friction with little additional pedal effort while the discs require increased effort just to maintain the same amount of friction. So the proportioning valve splits the applied pressure so each brake receives just the right amount.
Inside the proportioning valve is a spring-loaded piston that determines how much pressure goes fore and aft. Each valve is calibrated for a specific application by the vehicle manufacturer, so it's important to make sure you get the correct replacement valve for a customer's vehicle if the original valve is defective. The calibration of the proportioning valve is fixed and cannot be adjusted.
Some vehicles have "load sensing" proportioning valves that change rear brake metering to compensate for changes in vehicle loading and weight shifts that occur during braking. This type of proportioning valve has an adjustable linkage that connects to the rear suspension or axle. As the vehicle is loaded, ride height decreases and pressure to the rear brakes is increased. This type of proportioning valve can be found on many minivans, pickups and even some passenger cars (Ford Tempo or Mercury Topaz, and Ford Taurus or Mercury Sable to name a few).
Load sensing proportioning valves are adjustable, and must be adjusted correctly if they are to properly balance the rear brakes to the vehicle's load. The valve linkage is adjusted with the suspension at its normal height (wheels on the ground) and the vehicle unloaded. The adjustment bracket or linkage is then adjusted according to the vehicle manufacturer's instructions, which typically involves adjusting the linkage to a certain position or height.
Load sensing proportioning valves are also calibrated to work with stock springs. Any suspension modifications that increase the load carrying capability (installing helper springs, overload or air-assist shocks for example) may adversely affect the operation of this type of proportioning valve. Modifications that make the suspension stiffer reduce the amount of deflection in the suspension when the vehicle is loaded, which prevents the proportioning valve from increasing rear brake effort as much as it normally would.
A defective proportioning valve or one that is not adjusted properly can also upset brake balance. So if the rear brakes on a vehicle seem to be overly aggressive (too much pressure to the rear brakes), or the vehicle seems to take too long to stop (not enough pressure to the rear brakes), the problem may be a bad proportioning valve. Proportioning valves can be tested by installing a pair of hydraulic gauges (one on each side of the valve) to see if the valve reduces pressure as it should.