The fitting on the pushrod is the atmospheric vent, vacuum is constantly stored in the booster on both sides of the diaphragm, when you push the pedal air is allowed into the front side through that vent and since the other side still has vacuum it pushes on the diapragm. Most cars just have holes in the boot and a felt filter since the pushrod end of the booster is mounted inside under the dash, on a bus it's down in the dust and road splashiness so it needs a hose that leads to somewhere with clean air to suck.