What is the most likely cause if the rear wheels lock up during a quick stop?

• A. Worn brake pads
• B. Defective proportioning valve
• C. Insufficient brake fluid
• D. Broken brake lines

Answer: (B)

The most likely cause of the rear wheels locking up during a quick stop is a defective proportioning valve. The proportioning valve plays a critical role in a vehicle's brake system by ensuring that the braking force is distributed appropriately between the front and rear wheels. When a quick stop is executed, the weight of the vehicle shifts forward, which typically causes a greater amount of braking force to be applied to the front wheels.

If the proportioning valve malfunctions, it can fail to limit the brake pressure to the rear wheels, leading to excessive braking force at the rear. This can cause the rear wheels to lock up while the front wheels continue to slow down more effectively, thereby compromising stability and increasing the likelihood of skidding. Therefore, a defective proportioning valve directly contributes to the unsafe condition of rear wheel lock-up during abrupt braking scenarios.