If the master cylinder requires constant addition of brake fluid, what could be wrong?

When a master cylinder requires constant addition of brake fluid, it typically indicates that there is a problem with the integrity of the hydraulic system, specifically involving the piston cups within the master cylinder. If the piston cups are worn or damaged, brake fluid can seep past them, leading to a loss of hydraulic pressure and requiring frequent top-ups. This not only reduces the effectiveness of the braking system but could potentially lead to a complete brake failure if not addressed promptly.

In contrast, excessive brake pressure would not directly cause a need for additional brake fluid; it might lead to other issues like premature brake wear or overheating. Clogged brake lines could create pressure problems but would usually become more evident through symptoms like brake dragging or inconsistent brake performance rather than a need for more fluid. Worn brake drums might lead to more brake-related issues, such as poor braking performance, but they don’t impact the fluid levels in the master cylinder directly. Thus, the observation of needing to constantly add fluid points to the failure of the piston cup seals.