I have loss of air pressure on the brake system. The engine is a non runner. Is there an air valve which i can charge the system to release the brakes through a compressor is it situated anywere on the vehicle, or how can i release the rear brakes so i can move the vehicle on a rigid tow bar.

pete, July 2008

There is usually somewhere on the air storage tanks to connect an external source of air, ie a compressor.

ethelready, October 2008

Trucks used to have plug in connectors for cases like yours. They would usually be situated under the front bumper, or through the rad. grill.
I am told (I'm a bit out of date!) that they stopped doing this some time ago ('elf an' safety, or more reliable trucks?) so yours may be too modern.
Looks like you will have to crawl underneath and wind them off manually (compressing the actuating spring).
MAKE SURE YOU CHOCK THE WHEELS !

Mr. Toad, August 2008