What causes the necessity to keep topping up the master cylinder?

The necessity to keep topping up the master cylinder is primarily caused by a leak in the brake line or tube. The master cylinder is crucial for the braking system as it contains the brake fluid, which is essential for transferring the force applied to the brake pedal to the brakes themselves. If there is a leak in the brake line, the brake fluid will gradually escape, leading to a decrease in fluid level in the master cylinder. This drop in fluid level can compromise the braking system's performance, making it vital to regularly top up the fluid to ensure safe operation of the vehicle.

Other factors such as brake pedal pressure, excessive brake pad wear, or dirty brake fluid do not directly cause the need to refill the master cylinder. Instead, they may affect the overall performance of the braking system but do not contribute to a reduction in fluid levels within the master cylinder itself like a leak would.