Moisture in the hydraulic system can cause this problem. I had this happen on a Honda. Water droplets in the fluid turn to steam when hot, causing the same condition as when air is in the system. When it cools, the steam condenses back to water, and proper operation returns.
If you have flushed your system with fresh fluid, and still have these symptoms, try taking the slave apart to clean manually. The way this system is set up, the fluid goes from the line right out the bleeder, leaving the fluid in the cylinder largely undisturbed.
Once, just after flushing mine, I took it apart and found water droplets in the cylinder.
I miss round headlights.
"When I was a young man, I liked to race my horse...." - G.H.W.S. 1878-1962
MortimerSickle screwed with this post 09-20-2012 at 03:04 PM