| Bionicle,
I had the same thing happen last year not on a track but every time i went out on the bike brake disc got really hot and the rear brake started binding up. When i took the master cylinder apart inside there is a PTFE bush that the piston slides up and down in, problem was the PTFE bush had gone oval and the piston wasnt sliding up and down properly. so i made a new bush out of brass and have had the best brake i have ever had on the duke for over a year now and it has never needed bleeding since.
Stu | 
Threaded Mode