This happened to me as well. I lent the Coupe out @ the GPNW show last summer, my driver said brake pedal hit the floor. I've never been able to find the leak, but I bet it's the booster.
https://www.youtube.com/watch?v=W_nbOQIlAg0
Mark
www.mphspecialties.com
To: anzhonda600owners@yahoogroups.com From: mgpstearns@...
Date: Tue, 31 Dec 2013 08:23:39 -0800 Subject: [anzhonda600owners] Re: brake reservoir tubing
I had the same problem, I couldnt figure out where my brake fluid was going. I finally figured out I had the same problem as Bob with it staying in my brake booster after I started having fluid fill the breather hose from the top of the cylinder head (thank goodness for the inline check valve otherwise my engine would have been filled with Dot3) so I romoved the breather hose and capped the inlet/outlet. Then came the final final blow, we were at a local car show and cruise night and after going up and down the strip I found I had no brakes. I found brake fluid had overflowed from the Breather vent on top of my brake booster and had completely drained my reservoir. I limped home and removed the brake booster and just bent the tube to the master, refilled the system and bled it and I haven't lost a drop of fluid since. My brakes work better now that its a closed system again and the brake booster is sitting on the floor of my garage awaiting its fait whether to be rebuilt or sold. So be careful about your fluid disapearing into your booster it may eventually come to a messy end.