I doubt it's lines. Maybe, and just maybe, the rear hydralic hose that goes from the line to the rear might have a partial obstruction, and possibly the problem wouldn't show when bleeding the back brakes.
If the back shoes are adjusted properly, it seems as if you will have to play hit or miss, and start throwing money at the problem to troubleshoot. As I have said, the first place that I would throw money at is the master cylinder. If that didn't cure it, I would try a new rear hydralic hose. When I take a stance, I have the foundation on which to support my hunch. Otherwise, I'd keep it zipped, or say that I'm blindly guessing.
Despite the fact that the symptoms don't sound as if this is the problem, one thing that you might want to check--because it's free--is the tension on the master cylinder rod. There is not supposed to be any pressure on the master cylinder, zero lash. It's worth checking out becuause you don't have to get your wallet out.
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