That is, they're fail-safe: in their unpressurized state the brakes are set hard. Air pressure lifts the brakes, so any break in the air line stops the truck rather than leaving it flying down the road without brakes. Trains work the same way.
That is, they're fail-safe: in their unpressurized state the brakes are set hard. Air pressure lifts the brakes, so any break in the air line stops the truck rather than leaving it flying down the road without brakes. Trains work the same way.
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