Michigan State Brakes Certification Practice Exam

A defective proportioning valve is a critical component in a disc/drum brake system because it helps to regulate the distribution of brake fluid pressure between the front and rear brakes. During hard braking, it is essential for the front brakes to engage more forcefully than the rear brakes to prevent the rear wheels from locking up. If the proportioning valve is faulty, it may allow too much brake pressure to reach the rear brakes, leading to an excessive force being applied. This can cause the rear wheels to lose traction and lock up during hard braking, resulting in a loss of control.

In contrast, worn brake pads generally affect braking performance but may not specifically cause lock-up; low brake fluid levels can lead to brake failure but are less likely to cause the rear wheels to lock under heavy braking; and faulty brake lines could cause uneven brake application but do not directly manage the pressure distribution like the proportioning valve does. Thus, the malfunction of the proportioning valve is specifically linked to the scenario of the rear wheels locking up during hard braking.