Definition
A two-function valve used in pneumatic systems pressurized by a stored gas (typically nitrogen) that controls how the system is charged and how it is depressurized. In its build-up position, the valve allows gas from the storage bottle to flow into the system slowly and progressively until operating pressure is reached. In its vent position, it isolates the storage bottle and bleeds the downstream system pressure overboard so the system can be safely serviced or shut down.
Plain English
A single valve with two jobs: it lets pressure into the system gradually when you turn it on, and lets pressure out safely when you turn it off.
Context Anchor
Seen in aircraft brake system descriptions, especially when studying brake master cylinders or brake control valves.
Derivation
“Build-up” refers to pressure increasing in the brake line. “Vent” means to release or open a path out. Together, the name describes the valve’s two jobs: build pressure for braking and vent pressure when braking stops.
Why Pilots Care
On aircraft equipped with these systems, a stuck or mis-positioned build-up and vent valve can either trap dangerous pressure in the lines during maintenance or fail to deliver pressure when an emergency system is needed.
Analogy
It works a little like covering and uncovering the drain in a small pump: cover the opening and pressure can rise; uncover it and the pressure can escape.
Intuition Check
Do not read “vent” here as an air vent in the cabin. In this term, it means opening a path for brake fluid pressure to release back toward the reservoir.
Example Sentence 1
Before servicing the emergency brake accumulator, the technician moved the build-up and vent valve to the vent position to bleed off residual nitrogen pressure.
Example Sentence 2
If system pressure exceeds the limit, the build-up and vent valve opens to release the excess safely.