Braking excitement! Options

[Jamie]

Last weekend I drove freshly repainted Mistress into Lexington for display at a local car exhibition, and a few blocks from the site the brake petal went clear to the floor without notice! Yikes!!! Didn't pump up at all, and thank goodness for the hand brake. When the show was over and temps had cooled down, I was able to get her the 17 mi. home with brakes acting normal? Power bled the brake lines twice, and she drove normally twice during the week. Back to Lexington yesterday, and again the petal went straight to the floor in the city traffic, way too much excitement for me! The common denominator both times was high air temperatures, and again she behaved well on the return trip home when things had cooled down. There are no leaks at the master cylinder, fresh brake fluid levels still full, so I suspect that an 8 yr. old 19mm MC has failed, on a 1973 in otherwise excellent condition with 54K miles. What else does this 914 brain trust group think could be the problem? Should I just go ahead and order a new MC? (IMG:style_emoticons/default/WTF.gif)