Hey, not a problem on the time frame. Do what you need to do, I will be here.
First, I need to confirm some things we mentioned in passing previously, but since we still don't have it resolved, they move up the importance ladder.
First, Has the rear wheel or brake caliper been off between the time you got the bike and the time the problem showed up?
Is it correct that you re-built the MC to address this very problem, and the re-build did not fix it? If that is not correct, please explain what prompted the MC rebuild.
OK, now some new questions...........
Are you using DOT 5 brake fluid?
Is it doing it every time you ride now?
Does it have to be 2 up for it to happen, or has it happened when you were by yourself too?
How many times has it happened since the MC rebuild?
Now, some new suggestions,
Check the run out of the rear wheel bearings and rear disc run out.
Check the cap for the M/C to make sure the atmospheric vent is not clogged.
Make sure the rubber diaphragm inside the M/C is fully retracted when installed. It should be flat across the bottom.
Let me know!
We WILL get this beat!