A customer reports that their brake pedal slowly sinks when pressure is applied without any signs of leakage. What is likely the issue?

The most likely issue when a brake pedal slowly sinks under pressure without any signs of leakage is related to a worn primary cup. The primary cup, part of the master cylinder, is responsible for maintaining hydraulic pressure to the brake system. If this component is worn or damaged, it can allow brake fluid to bypass it, leading to a gradual loss of pressure. This results in the brake pedal sinking slowly as the pressure is applied.

In scenarios where there's no visible leakage, it suggests that the fluid is not leaking externally but is instead being allowed to flow back past a failing seal within the master cylinder, which corresponds with the primary cup's condition. Maintaining the integrity of this component is essential for a firm and responsive brake pedal.

Understanding this issue is crucial, as it emphasizes the importance of inspecting the master cylinder and its internal components whenever a customer reports similar symptoms. This highlights the need to prioritize internal wear and seals during brake system diagnostics.