Under hard braking, if the rear wheels lock up on a vehicle with a disc/drum brake system, what is the likely cause?

The situation described involves the rear wheels locking up during hard braking, which is indicative of an imbalance in the braking forces applied to the front and rear wheels. In a disc/drum brake system, a proportioning valve is responsible for regulating the pressure to the rear brakes in relation to the front brakes.

When the proportioning valve malfunctions, it may deliver excessive hydraulic pressure to the rear brakes compared to what is necessary, leading to a situation where the rear wheels lock up before the front wheels during hard braking. This not only affects braking performance but can also lead to instability and a loss of control of the vehicle.

Understanding the role of the proportioning valve is crucial because it ensures that the rear brakes do not contribute excessive stopping force under hard braking conditions, which can cause losing traction at the rear. A faulty brake master cylinder could lead to issues like weak braking performance generally, and worn brake pads would typically cause longer stopping distances or increased pedal travel rather than locking up the wheels during braking. Overinflated tires may affect traction but do not directly cause the rear wheels to lock up in a braking scenario. Thus, the most accurate cause of this issue would be a defective proportioning valve, which could disrupt the intended brake force distribution.