How close to your exhaust pipe is the brake line? It sounds to me like the fluid is boiling in the line.
edit: Perhaps I should rephrase. I had to get creative with the plumbing when I swapped out the stock master for a dual-resevoir power unit (roughly 4" offset to fit the booster under the floor). The stock exhaust was suddenly dangerously close to the lines, I managed to reroute them but I was already fabricating a heatshield in my head...
On my truck the tailpipe comes out just behind the left rear wheel. Is yours still hanging where it should be?
