A car's wheels severely vibrate when the brakes are applied. Which of the following is the least likely source of the problem?

The master cylinder is primarily responsible for generating the hydraulic pressure necessary for the brake system to function. While issues with the master cylinder can lead to other braking-related problems, it does not directly cause vibrations in the wheels when brakes are applied. Severe wheel vibrations typically arise from problems that affect the mechanical aspects of the braking system or the wheels themselves.

Worn rotors can lead to uneven braking surfaces, causing the brake pads to grip inconsistently and thereby producing vibrations. Unbalanced wheels can create vibrations during driving, especially when the brakes are applied, as they can exacerbate any existing imbalances in the system. Warped brake pads might also create pulsation or vibrations since they would not maintain a consistent contact surface with the rotor.

In contrast, while all components are integral to the braking system, the master cylinder’s primary function does not directly relate to the physical vibration of the wheels during braking. This makes it the least likely source of the problem in this scenario.