I have been using Avid Juicy 7's for almost about 18 months without any problems but on my last 3 or 4 rides the rear pads tend to drag after long downhill sections (read lots of braking). I guess that the fluid heats up and therefore expands after lots of braking but I never had the dragging problem before. Once the brakes cool a bit they stop dragging again. The fluid is original (18 months old) so I am wondering if it needs to be changed? What is the life of disk brake fluid? What else could be causing the brakes to drag when hot?