from e looks of e DM3500 incident, it's likely either e air
brake failed because of sudden leakage in air pressure tank or not
enough 'bar' of air pressure.
From a techinical perspective, the brake systems in heavy
vehicles are normally fail-safe and sudden drop in air pressure
would not have caused the accident.
Pressured Air is used to push the brake pads (in the case of
disc brakes) and brake shoes (in the case of drum brakes) to create
friction to stop the vehicle.
Lack of pressure in the air would no doubt cause the brakes to
become ineffective, but however, there is a backup mechanism.
Thats where the handbrakes come in, instead of using the
Pressured Air to push the brake pads/shoes to create friction, the
handbrake uses the Pressured Air to pull the
brakes away from the wheels. Therefore if the air pressure is not
enough, the vehicle will also be unable to move.
If you remembered, there was one incident few years ago where a
volgren bendy had to be towed back due to problem with the air
pressure, the vehicle was stuck and unable to move until nuts was
installed to release the brakes
