If a technician observes a swollen diaphragm in the master cylinder cup, what is the likely cause?

A swollen diaphragm in the master cylinder cup typically indicates a reaction to the properties of the fluid within the brake system. When silicone brake fluid is added to a system that is not designed for it, the diaphragm material can swell due to chemical incompatibility. This can cause the rubber components of the master cylinder to become ineffective, leading to potential brake failure.

Silicone fluids are not universally compatible with all types of brake system components and can lead to physical changes like swelling in materials that aren’t engineered to handle its chemical structure. This issue is important for technicians to recognize, as it points towards improper maintenance practices or the use of the wrong type of fluid, which can have serious safety implications for vehicle braking systems.

Other options may relate to different issues, but they do not directly explain the characteristic swelling of the diaphragm in this context. Low fluid levels might lead to other symptoms, like air entering the system, while contaminated brake fluid could cause various issues as well, but swelling is most commonly associated with the inappropriate use of silicone fluid. Normal wear and tear generally does not present as swelling but rather as general degradation or deterioration of materials over time.