My buddy and I went riding today. Me on my 450SX and him on his 450EXC. About 2 hours into the ride his clutch goes south. It breaks down like this. I am on a steep and extremely muddy hill and stall my bike while trying to turn hard right to get on a different trail. My buddy has to come to a stop and when he starts up again the clutch feels funny. By the time we got to the top of the hill all of his fluid was gone (we checked and it was bone dry). We limped back to the cars and when he got home he couldn't see any damage to the lines, the master or slave cylinders. We didn't have any fluid to pour back in to see if there were any leaks. He did just finish putting on the slave cylinder protector and he bleed the clutch during that process. But it was fine for several days and several hours worth of riding. Any ideas?