400i brake problem...

Our 400i was dripping brake fluid off the rocker in front of the left rear tire. The fluid was dripping out the small square frame tubing behind the rocker. I searched for the source and I assumed it was coming down the firewall and finding it's way to the square frame tubing. My assumption was that the master cylinder had an internal leak that was finding it's way out through the booster... I had the M/C rebuilt, it required sleeving. What's the chance the rebuild would fail in an identical way after sleeving? The only other thing I can think of is that a brake line has rusted through, but that seems kind of extreme for a California car...