What could be a reason for experiencing a vibration or pulsating effect when brakes are applied?

Experiencing a vibration or pulsating effect when brakes are applied is often attributed to rotor thickness variation, which is a condition where the thickness of the brake rotors is inconsistent. This can result from factors such as excessive heat during braking, uneven wear, or warping of the rotor. When the brake pads clamp down on a rotor with varying thickness, the pressure applied is uneven across the surface, leading to a pulsing sensation. This not only affects braking performance but can also lead to further wear and damage to both the rotors and brake pads over time.

Other factors like a brake line leak, worn brake pads, or brake fluid contamination might lead to brake issues, but they typically do not cause a pulsating feel during braking. For instance, a brake line leak could result in brake failure or reduced braking force, while worn brake pads may produce a squealing or grinding noise rather than a pulsation. Brake fluid contamination can affect the hydraulic system's efficiency but would not directly cause the vibration felt through the pedal. Thus, rotor thickness variation is the most accurate reason for the specific symptom of vibration or pulsation during braking.