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I just checked the Air line diagram in the TM, it looks like the two way check valves under the drivers step is likely where the problem lies. The one behind the bumper is for the emergency gladhand. Here is a picture from the manual hope this helps.

No, this has been discussed in prior threads, I believe there is a check valve behind the bumper that is the cause. My truck was leaking at the red glad hand, in that case the shuttle valve under the step needed replaced.