Originally Posted by Gatorbait
So I have a 2003 Thomas/frieghtliner FS65 shorty. It has airbrakes. I want to hook up an air ride drivers seat. I was just going to tap off the hose I disconnected from the stop sign. I'm not sure what size hose that seat is going to require, but if I have to run a bigger hose I'd have to release the pressure in the tank to put a fitting on. How exactly do the air brakes work? If I release the air in the tanks, will the brakes release? I don't have a "park" on my transmission. It's just neutral and the air emergency break. My wheels will be chaulked obviously, but I'm curious if you were parked on a hill and there was a leak in the system. It seems as though there'd have to be some kind of fail safe. Does the lever close a valve and keeps the pressure to the brakes? sorry im rambling.
The air pressure in an air-brake system holds the brakes "Off". If the pressure is released, the brakes engage.
There are two brake systems in play. A proportional system that works when you press the brake pedal (it's actually a valve), that applies the brakes progressively.
The parking brake is what is called a "Spring brake". Air pressure holds the brakes off until you shift the yellow brake valve. That releases the pressure that is holding back an extremely strong spring. When the spring is released it engages the rear brakes.
You need an air pressure of between 90 and 120 psi for the brakes to work effectively. Below 60 psi the spring brake will engage and you are not going anywhere.
There should be a valve on the bottom of the air cylinder under the bus where you can manually release the pressure if you need to. It's also used to blow out any water that might have collected.