Brakes are designed to stop only so much energy (so quickly) before they overheat. Whenever I am climbing up a hill, and I am watching the tranny temp climb, I am even more afraid of the downhill. Those poor brakes have to slow down all that stored energy I just put into the rig.
I remember once when I was at Extreme, there was a guy who had a small boat, but kept burning up the brakes coming down from Big Bear. He had to run the biggest disks possible on all three axles just to keep from melting the brakes.
Just make sure that you aren't constantly engaging the brakes, which will give them a chance to cool down slightly.