the brakes are actually quite good if u set them up right , i thought they were crap as well ,but put in the time seting them up and they work fine ,i even managed a front flip with the standard servo , after i played with the linkage [going to servo] there is also adjustment on the link [ the one in which u can adjust the leverage or turning radius if u like] spend the time and u'll get it right , as cenracer1 suggests a high torque servo will make a massive difference ,but again if ur setups not right ,u might aswell piss against the wind .Check that ur disc has no fuel spilt on it ,cause again if it has its all greasy a needs to be cleaned or it wont grip ,first thing to do is clean it [use some kind of degreaser, i did a it made a big difference] then go at ur linkages.
hope this helps