How does the Yamaha Grizzly wet brake system work?

I have recently purchased a Big Bear with a wet brake and have been having significant problems with the rear differential housing getting so hot that I cannot keep my hand on it even after riding in minus 12 degree celcius weather. It seems that the brake consists of five disc plates that sit inside the housing beside the differential gears and are therefore always bathed in the 80 grade heavy gear oil. Even under that condition and during times when the brake is not applied, the housing gets hot. Initially it got so hot that the rear end had to be rebuilt. Yamaha Canada suggests that heat up to 60 degrees celcius is acceptable -- however the mechanics and dealers that I have talked to say there should only be a slight amount of amout of heat (warm to the touch) building up in the same way that a convential differential operates. I am curious to know if others are having heat build up too. joehunwicks@hotmail.com