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Incorrect!

What could prevent air pressure from building up in an air-brake system in frosty weather?

Moisture in the air may form bubbles in the brake fluid
The air will contract, reducing the pressure
You selected this answer
The dampness may cause valves to rust
Moisture drawn in with the air may freeze and cause a blockage
Correct answer

DVSA theory test 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.

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