Air Lines

[TheSDTrucker]

I crawled under a trailer today to get an idea of what the air line plumbing looks like on a trailer. I am having trouble distinguishing which line does what. I noticed there are two different sizes. Which one would be the hose for the red gladhand and which one would be for the blue gladhand? 

[dad vader]

The smaller hose(usually 3/8 inch nylon hose) is for the spring/parking brake system. The larger hose(usually 1/2 inch) is for the service side They both go to some kind of relay valve which applies tank air to the systems.