Definition
A small opening in the master cylinder of a hydraulic brake system that connects the fluid reservoir to the cylinder bore when the brake pedal is released. It allows brake fluid to flow between the reservoir and the cylinder to compensate for fluid expansion, contraction, and minor losses, keeping the system filled and at the correct pressure when the brakes are not applied.
Plain English
A tiny hole inside the brake master cylinder that lets brake fluid pass back and forth between the reservoir and the cylinder when your foot is off the brake. This keeps the system topped up and prevents pressure from building when it shouldn't.
Context Anchor
Seen in aircraft brake system maintenance, especially when checking for dragging brakes, locked brakes, or brakes that do not release normally.
Derivation
From 'compensate' (Latin compensare, 'to balance or make up for') plus 'port' (Latin porta, 'opening or gate'). The name describes its job: it is the opening through which the system balances itself out when the brakes are released.
Why Pilots Care
If the compensator port becomes blocked or the master cylinder piston covers it when the brakes are released, fluid cannot return to the reservoir. Heat expansion then builds pressure in the brake lines and the brakes can drag or lock up, which causes wheel and tire damage and can lead to loss of directional control on the ground.
Intuition Check
Do not think of port here as an airport or a computer plug. In this brake-system context, a port is a small fluid passage.
Example Sentence 1
When the mechanic found the brakes dragging after taxi, he suspected the compensator port in the master cylinder was blocked.
Example Sentence 2
A blocked compensator port allowed fluid expansion on a hot day, producing unwanted brake drag during taxi.