My brakes get mushy under heavy pressure I have some Avid Juicy 5 on my Specialized SJ FSR '08 and the front brake gets mushy and looses brake power on long downhill trails.

Do I have to purge the brake lines to get performance back on track?
As @Batman has suggested brake fluid is hygroscopic. Over time moisture from the air is absorbed by the fluid. When the brakes get hot enough the  absorbed water will boil. This results in air bubbles forming in the fluid. While brake fluid is not compressible the air bubbles are. This results in a mushy or spongy feeling to the brakes. Once cooled the bubbles may be reabsorbed allowing the brakes to feel normal.
+