My truck has a brake line that exits the coupler by rear end and goes no where

What is your question?

I have a brake line that appears to be for brake assist for hauling a trailer. It is connected to the coupler near the differential and heads up over the gas tank but does not come out anywhere. I would not be concerned but it has started to leak. Any ideas on what this is and how to repair. I do not haul large trailers with this truck and would cap it off if not essential to the braking of my truck. Thanks