Don't forget to set the parking brake.

Whoops.

http://www.youtube.com/watch?v=_vE9Zq7Gs-U&feature=player_embedded

that's weird. air braked vehicles use air to keep the brakes from engaging (opposite of hydraulic) Without the engine running, those brakes should have been locked tight

Every action has an equal and opposite reaction. ProoF!

mad_machine wrote: that's weird. air braked vehicles use air to keep the brakes from engaging (opposite of hydraulic) Without the engine running, those brakes should have been locked tight

actually, they use air to disengage the parking brake, so that you can set the parking brake even if the air source fails. problem is, you don't always think to set the parking brake when you're climbing UP out of the side window.

Strizzo wrote:

mad_machine wrote: that's weird. air braked vehicles use air to keep the brakes from engaging (opposite of hydraulic) Without the engine running, those brakes should have been locked tight

actually, they use air to *disengage* the parking brake, so that you can set the parking brake even if the air source fails. problem is, you don't always think to set the parking brake when you're climbing UP out of the side window.

That and a lot of those systems will hold air for days without the engine running. The good thing is his air system doesn't leak, the bad, he didn't pull the yellow knob on the way out.

What nobody seems to know and what everyone should is what Strizzo explained. Semi Trucks use air to disengage the brakes.

In other words: IF THERE AIN'T NO AIR IN THE SYSTEM YOU AIN'T GOIN NO WHERE.

....that's gonna be some extra paperwork....

Railroads use air pressure to release brakes. Road brakes on a semi are engaged by air pressure. Not sure how parking brakes work, but am certain that they use pressure to apply brakes.

The parking brakes in semis are released by air, the regular braking is engaged by air, so there are two braking circuits in play. Here is a description from the interwebs on the parking brake:

The parking brake of the tractor unit and the emergency brakes of the trailer are spring brakes that require air pressure in order to be released. They are applied when air pressure is released from the system, and disengaged when air pressure is supplied. This is an emergency feature which ensures that if air pressure to either unit is lost, the trailer will stop to a grinding halt instead of not stopping and becoming uncontrollable.

Driving an air-brake equipped vehicle everyday, I can say that yes, the normal brakes need air to engage, the parking brake needs air to disengage. On my truck it is approx 70 psi to disengage, though the system gas a horrid warning beep if it goes below 70. We have an older truck at work with air brakes, and a cable actuated emergency brake. Much smaller though.

I dont really care how or why I just think its funny as hell.