Tau Tau 110 cc rear brake problem

I bought my son this ATV a couple of years ago it has been a great machine.The only problem it has had since purchase is the rear disc brakes seem to require constant adjustment. They seize on and require an adjustment to make the machine move again. I assume that the problem lies in the return port on the master cylinder. But I wonder if the exhaust is too close to the master cylinder and causes the fluid to heat up applying the rear brakes. Any similar problems out there.  

possibly dust and crud building up?  

Hello,
 It does sound like a dirt or gunk problem. it is not likely that you have a fluid return problem. Be careful what kind of spray or cleaner you use on your caliper. WD40 and others like it attract dirt and can cause a sticky residue. Disks are usually trouble free as opposed to drum brakes as they are usually not affected by heat and dirt. Make sure that you system is bled of all air and that you are using a good grade of brake fluid. The higher the rating the better. Especially look for dirt accumilation in your lines/resevoir. Make sure that the rubber boot on your caliper is not torn or leaking. You may want to try a good lithium grease on the outer contact of your piston area.
Good luck.