A more detailed look at the air braking system, in particular with this video at the air brake relay.
Relays are fitted to an air braking system for larger vehicles to speed up the application and release of the brakes, minimising brake lag, which is the time it takes between the driver operating the controls (park brake/foot brake valve) and the action actually happening at the brakes.

For a bit more background before watching this video, take a look at our "introduction to air brakes"    • Air Brakes - An Introduction. How it ...  

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