What could cause a brake pedal to feel hard?

A hard brake pedal often indicates that there is an issue with the brake assist system, primarily the vacuum assist or brake booster. The brake booster uses vacuum pressure to aid the driver in applying force to the brake pedal. If there is a malfunction in this system—such as a vacuum leak, a faulty booster diaphragm, or a restriction in the vacuum supply—the assistance provided during braking will diminish. As a result, the driver will need to exert significantly more force to engage the brakes, making the pedal feel hard and unresponsive.

In contrast, problems with brake fluid quality typically lead to a spongy pedal feel rather than a hard one. A malfunction in the brake master cylinder could contribute to various issues, including fluid leaks or inconsistent braking, but it doesn't specifically produce a hard pedal feel. Worn brake pads mainly affect braking performance and noise, but they do not usually cause the brake pedal itself to feel hard.