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I've found in different trucks and buses, etc.... An "H" button which, when pressed, activates a braking system. This system is only available when the vehicle is immobilized.
Which brakes are activated? How does it work? Why do they stay on even if the button is deactivated and until the driver makes an action to restart motion?

Whenever I try to search for this I continuously get unrelated answers about how brakes work. I just want to know about that subsystem.

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That'd be the handbrake (also known as parking brake or emergency brake), in lieu of a traditional cable-operated brake - the latter can't be used on an air-braked vehicle such as a truck or bus.

There will be a valve somewhere, operated by that button, that locks the brakes on when that button is pressed - usually using a spring to apply the brake and air pressure to release it again. When the driver goes to restart the vehicle, the valve will be closed again allowing the air pressure to release the brakes. This also acts as a safety mechanism, as if the air pressure is suddenly lost (e.g. by the trailer breaking free), the spring will re-apply the brake and stop the vehicle...

  • Are you sure you aren't confusing this kind of brake with the parking brake? The "H" one is using instead of the parking brake. It also must not be used as if it was a parking brake as it doesn't work well (or at all, I'm not sure) if the engine is not running. – brunoais Dec 11 '14 at 20:17
  • So is this button present as well as another control for the handbrake? I assumed from the way you'd written it that it was an electronically activated parking brake (as per many modern cars)... In that case, perhaps it's some kind of hill-assist brake? i.e. hold the brakes on so the driver can release the pedal to apply power... – Nick C Dec 12 '14 at 9:13
  • AH, I c... So it is the parking brake except it is electronically controlled such that, if the user deactivates it, it goes ready to help start motion. Anyway, if you want to, I can get you a shot of it. Do you want to? – brunoais Dec 12 '14 at 17:28

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