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Air blowing out yellow lines at outside firewall when setting trailer brakes

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Got a 2012 chu613 that when you set the trailer brakes air blows out the yellow exhaust line on the firewall for several seconds then the trailer brakes pop. Air continues to vent at the firewall for a while. I’ve been working on a large fleet of these and I remember having this problem a long while back but can’t remember what it took to fix it. Oh and also the tractor park brake don’t apply until a lot of air escapes. So when you pull both brake buttons air rushes out the firewall vent lines for several seconds until you here a dump at the trailer then all brakes apply

Edited by c-mac

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Turned out to be the “dump” valve on the trailers air tank.

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