[cripplecreek]

On a truck, as the air bleeds off a spring brake locks down.

[thackney]

On a truck, as the air bleeds off a spring brake locks down.

Trains are different.

The air pressure that holds the brake open also supplies are to pressure up individual tanks on each car to initially hold the brake closed. It is intended to give enough time to allow the hand brakes to be set on each individual car. It is not designed to stay held closed forever.