Like J Blaze stated I would tighten the screws on the brake calipers and/or adjust your brake linkage first before spending money on a new servo or battery. Also check your End Point Adjusments on your radio.
If you race, you really don't want too much brake especially when entering turns. The last thing you want during a race is to lock up the wheels making the rear slide loose and you end up spinning your truggy out.