Does it have a proportioning valve if so read something about it see below could very well be the ABS
PROPORTIONING VALVE & BRAKE BALANCE
To reduce hydraulic pressure to the rear brakes so the rear brakes don't lock up when the brakes are applied, a "proportioning valve" is required. This valve helps compensate for the differences in weight distribution front-to-rear as well as the forward weight shift that occurs when the brakes are applied.
What we're really talking about here is "brake balance" or "brake bias, " which is the difference in the amount of hydraulic pressure channeled to the front and rear brakes. The front brakes on most rear-wheel-drive vehicles normally handle about 60-70 percent of the brake load. But on front-wheel-drive cars and minivans, as well as RWD and 4WD pickups and SUVs, the percentage handled by the front brakes can be as much as 90 percent of the load.
Consequently, the front brakes need a higher percentage of the total hydraulic force that's applied to keep all four brakes properly balanced.
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 pulls the shoes up tighter only against itself. Because of this, drum brakes that are "self-energizing" require little additional pedal effort once the brakes are applied. Disc brakes, on the other hand, are not self-energizing. It takes increased pedal effort to squeeze the pads against the rotor.
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.
Load sensing proportioning valves usually 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, or 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 properly adjusted, can also upset brake balance. 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.
On some late-model vehicles, the mechanical proportioning valve has been replaced by "electronic" brake proportioning through the ABS system. By sensing wheel speeds, the ABS system reduces pressure to the rear brakes as needed when the brakes are applied.
Sunday, November 25th, 2007 AT 5:20 AM