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Hello. Welcome to Just Answer. Please allow me to assist you. The third line is a vent line. You have a vacuum line, an engage line and a disengage line. That is all that is required to move the vacuum operated shift motor on the transfer case. The remaining line is a vent. It vents the lines to the atmosphere as they are in between modes (like when being switched vai the knob in the glove box. The ' T ' is missing another small line that was routed into the inner fender area where the vent terminates. You are missing that connection to the inner fender.
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