Q: What could prevent air pressure from building up in an air-brake system in frosty weather?
Explanation
When air is compressed, moisture condenses and collects in the air tanks. This can find its way along the network of pipes connected to the brakes. In frosty weather, the moisture can freeze in the pipes, blocking them completely. On modern vehicles, the air is dried before it's compressed and the air tanks drain automatically to shed any moisture. On older vehicles, the air tanks need draining manually.