C152 instructor reports right brake failure while the student was attempting to taxi the aircraft. Instructor took control; but was unable to stop before going off taxiway.

Narrative

This flight was the 2nd training flight for the student. Preflight and engine startup were normal. No brake fluid leaks were detected upon external inspection; but the outboard brake pads on both sides were noted to be close to end of normal life. We both; the student (left seat) and I (right seat); tested the brakes before calling Ground for taxi instructions. We were cleared to taxi from the North ramp to Runway 6 via Bravo then Alpha. The taxi proceeded normally with the student learning rudder steering while I followed closely on the controls. We stopped short of the holdshort line at Runway 6 while in a slight left turn. While holding short of Runway 6 at Alpha; we ran the Before Takeoff Checklist. On this lesson; I let the student run the entire checklist with myself clarifying and explaining as necessary. The Before Takeoff Checklist was completed normally; but the time period took longer than usual since the student was working the checklist and this was only his 2nd lesson. We then called Tower and were cleared for takeoff Runway 6 with a left turnout for a North departure. As we began rolling to taxi from the holdshort line to the runway; the student had difficulty straightening the aircraft toward the runway as the aircraft stayed in a left turn. I immediately took control of the aircraft. I attempted full right rudder pedal and right brake. The rudder attempted to respond; but the right toe brake went horizontal from lack of brake pressure. The aircraft continued to turn left as rudder pressure alone was insufficient to turn right and straighten the aircraft. I now reduced the throttle to idle and attempted full brake on both sides to stop the aircraft from rolling. This had the effect of tightening the left turn. The nosewheel of the aircraft rolled off the taxiway. With the aircraft now slopping slightly downhill (because the nosewheel was off the taxiway); I had difficulty bringing the aircraft to a stop. I pulled the mixture to kill the engine. The aircraft came to rest with the nosewheel off the taxiway; still short of the holdshort line to Runway 6. As the aircraft was sitting stopped; I pumped both brakes several times and could feel brake pressure return somewhat to the right brake pedal. From my seat I could see the right brake pads move when right brake pressure was then applied and released. Upon external inspection; the brakes appeared normal with no hydraulic leaks. There was no damage the aircraft or the airport. There were no injuries. I speculate that there is a hydraulic leak within the right brake cylinder; and the extended period of holding the brakes caused fluid to gradually move behind the cylinder head. This would cause a loss of hydraulic brake pressure and the resulting brake failure.