The second line called service line, its normally blue colored or with blue fittings, and it’s usually smaller than the other one.
When you are performing a normal breaking, pressing the brake pedal will pressurizes the service line. Whit that happening, the valve in the trailer is activating and will direct air from the reservoir and the emergency line to the brake cylinders.
With that the piston moves and activates the brakes.
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If you let the pedal up, it will decrease the service line pressure.
With the drop of the service line pressure, the valve in the trailer will block the air supply from the reservoir.
At the same time will release the pressure in the brake cylinder, and release the brakes.
So this is exactly how do Air brakes work! The system is something like a servo or amplifier.
