how do you bleed lorry air brakes?

Probably 2 bleed nipples on each wheel hub. Remove little rubber cap (if it's still there!). Clean dirt away from bleed nipple. You need a one man bleed kit if you are doing this on your own. Push end of hose/tube (part of kit) over bleed nipple and open nipple using the appropriate size wrench (13mm probably). In cab depress brake pedal repeatedly until the fluid coming out of the bleed nipple is free from air bubbles. When air is gone, close bleed nipple and remove tube. Put some rubber grease (special red grease for hydraulic brake components) on the rubber cap before placing back on nipple.
Keep the brake master cylinder or cylinder(s) reservoir topped up with brake fluid while doing this.
Wear appropriate ppe - i.e. latex gloves and eye protection as the brake fluid is highly corrosive and you don't want it on your skin or in your eyes! (make sure bleed tube is properly attached to nipple - high pressure can blow it off, spraying fluid everywhere).

JD
April 2008