What is the most probable cause of excessive dragging left brake on an airplane with no recent service work?

The most probable cause of excessive dragging of the left brake, particularly in an airplane that has not undergone recent service work, can be attributed to foreign particles stuck in the master cylinder compensating port. When contaminants obstruct this port, it can prevent the proper flow of brake fluid, leading to a condition where the brake does not fully release after application. This effectively keeps the brake partially engaged, causing the dragging sensation.

The compensating port's function is crucial for allowing fluid movement between different parts of the braking system, thereby ensuring that brakes can release fully when not in use. If this port is blocked, it disrupts the hydraulic system's normal operation, leading to unwanted braking action.

While worn brake pads, low brake fluid levels, and a faulty brake actuator can indeed cause various braking issues, they may not specifically relate to excessive dragging in the absence of recent service work as directly as blockage in the compensating port does. Worn brake pads would typically manifest as a general decrease in braking efficiency, low brake fluid could lead to a complete failure of braking action rather than dragging, and a faulty brake actuator would usually result in inconsistent or unresponsive braking rather than a constant drag.