So, I called Sunset and they said to send the whole brake back to them. I knew it was the master cyl, as it had LHM leaking from it. When you looked down the lever into the area where its piston enters the cylinder, a squelchy noise came out and LHM. New seals cost about a fiver, but they said they'd do it on the Warranty.
No MTB until the post it back, though.