A cattle chute with one-way gates allows the cows to only move in one direction – forward.
A one-way check valve in an air brake system works exactly the same – it allows air to only move in one direction – forward through the system.
The one-way check valves are located at the entrance of the primary and secondary air tanks, and these are a major fail-safe of the air brake system. These are required to be checked as part of the driver's pre-trip inspection.
As well, there is one located at the trailer air tank.
The one-way check valves at the entrance of the primary and secondary tanks are primarily responsible for dividing the system into two independent subsystems. If there is a leak in either subsystem, the one-way check valves prevent air from bleeding out from the subsystem that still maintains air pressure.
To keep the parking brakes off, the two-way check valve is designed to pull air from the system with the higher pressure; therefore, the spring parking brakes will not apply automatically in the event that one of the subsystems fails.