Prior to start-up, the student pilot performed a preflight inspection while the flight instructor reviewed the airplane maintenance discrepancy sheet. The flight instructor noted that the airplane had recently been written up for inconsistent right brake pressure. Maintenance personnel resolved the discrepancy by tightening the pilot side brake pedal and returned the airplane to service. Everything appeared visually normal during the subsequent preflight inspection, and the student and instructor performed an engine runup with no braking problems noted. During the subsequent taxi to the runway, while attempting to negotiate a turn, the right brake was not effective despite both the student and the instructor applying brake pedal pressure. The airplane went off the taxiway and ultimately struck a tree resulting in substantial damage to the horizontal stabilizer. A review of photographs taken by an FAA inspector of the disassembled braking system components revealed that the right brake master cylinder o-ring appeared worn and possibly had a flat spot. It is possible that the wear/flat-spot of the o-ring could have allowed fluid to bypass it, which would have resulted in the inconsistent brake operation described by the pilots.