Correct answer
To prevent overpressure in the system.
It protects against excessive pressure by releasing air above its specified opening pressure. It is distinct from a quick-release valve, which exhausts brake-chamber air when braking ends, and from the reservoir drain valve used to remove condensation. These have different functions. Follow system specifications and maintenance; do not adjust or disable the valve to raise pressure.
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To allow air to pass into the semi-trailer circuit.
To automatically bleed air from the hydraulic fluid.
To prevent overpressure in the system.
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