Hey Andy,
My master started to do that same sort of thing
after I put on a front wheel line lock.
I got it to stop by really bleeding the lines
and making sure all the old fluid and air was out of the system. I also realized that using the line lock which puts up to 3000 PSI on the front break lines really heats the fluid
up. I found that even
a small amount of air in ths lines expanded and
back-flowed fluid into the master and it leaked out of the top of the front res.
Hope that gives you a clue.
Steve (sageman)

If it isn't one problem it's another. I'm having a problem with my 1200's brakes. In June, I put a new master cylinder in it for a newer 210/120Y. Anyways, recently it started losing
fluid from the rear resevoir, and it looks like it is going into the front resevoir and then over filling the front one and leaking out all over the place. Is it supposed to do this or is
my master cylinder screwed up somehow? It's starting to really rust up my engine compartment so quick advice would be greatly appreciated. Thanks

-Andy