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Farm Machinery
Machinery
Attaching trailer air brakes
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<blockquote data-quote="Andrew" data-source="post: 8157357" data-attributes="member: 93"><p>On most ag and older hgv trailers the brake actuators are sprung OFF. </p><p>The red line fills the air tanks on the trailer. The yellow line acts as a signal to the RELAY valve that uses air pressure from the air tank to apply the brakes. The yellow line does not apply the brakes. The relay valve also senses any drop in pressure in the red line and applies the brake if so using the air pressure from the air tanks. </p><p>This is why when you disconnect the red line you have brakes on, but drain the air out and the brakes will release - which is why a mechanical handbrake is a legal requirement.</p><p>A shunt valve can also be fitted, which sends air from the trailers tanks into the red line port of the relay valve, fooling it into thinking the airlines have been connected and releasing the brakes.</p><p></p><p>More modern HGV trailers use spring ON brakes, which have an extra port. Air pressure is needed to release the brakes, but then the system generally works as above. </p><p>This negates the need for a separate mechanically appllied handbrake.</p></blockquote><p></p>
[QUOTE="Andrew, post: 8157357, member: 93"] On most ag and older hgv trailers the brake actuators are sprung OFF. The red line fills the air tanks on the trailer. The yellow line acts as a signal to the RELAY valve that uses air pressure from the air tank to apply the brakes. The yellow line does not apply the brakes. The relay valve also senses any drop in pressure in the red line and applies the brake if so using the air pressure from the air tanks. This is why when you disconnect the red line you have brakes on, but drain the air out and the brakes will release - which is why a mechanical handbrake is a legal requirement. A shunt valve can also be fitted, which sends air from the trailers tanks into the red line port of the relay valve, fooling it into thinking the airlines have been connected and releasing the brakes. More modern HGV trailers use spring ON brakes, which have an extra port. Air pressure is needed to release the brakes, but then the system generally works as above. This negates the need for a separate mechanically appllied handbrake. [/QUOTE]
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Farm Machinery
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Attaching trailer air brakes
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