if your wearing riding boot you may be putting the brake on
put a buzzer on the dash hook it up to brake lamp and go for a ride and see if its buzzing when your riding and not braking
i trashed a set of pads on the rt 125 when i first got it riding the brake when i did not mean tooooo
also check the leaver adjustment the lever may not be returning back far enough when you release
it and the fluid gets hot and expands and puts the brake on because excess fluid can not return to the master cylinder reservoir
65 years working on bikes , two-way radios and mechanical and digital computers = if something is not farked up ...just wait a few minutes
here is my favorite piece of "KIT" working on one of these divides the men and boys by there talent
all the fellows i know that were good at fixing them did not do math very well
http://www.vcalc.net/cu.htmthis is a very interesting read