What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
Pilot initiated taxi, with the student at the controls. Initially taxied without any problems, but pulling onto taxiway “f” the student stated that the rudder controls started to feel weird, and the student responded by pulling the throttle back to idle. Ground speed indicated roughly 10 knots at the time of throttle idling, and subsequently started reducing. Immediately after the airplane started pulling to the left, and the student quickly informed instructor that he was losing control of the airplane. Instructor took the control of the airplane and applied full right rudder, while applying full right brake. Instructor stated rudders and brakes felt mushy, and yielded no response resulting in the airplanes pull to the left. The airplane moved at a slow rate of speed while slowly drifting off the taxi way centerline. Eventually the airplane came to a stop slightly off the taxi way, at which time ground control was notified.
After the airplane had come to a complete stop, the instructor proceeded to pump on the brake pedal, and after a few pumps he felt the pedal starting to resist and the right brake came back to life. This allowed him to realign the nose towards the taxi way centerline, but with safety in mind the decision was made to get the airplane towed back to the ramp, rather than attempting taxiing back.
In conclusion they inadvertently lost control of the airplane due to a temporary loss of right brake effectiveness. No property, person or airplane was damaged during this event.
As the FAA filed it
PILOT INITIATED TAXI, WITH THE STUDENT AT THE CONTROLS. INITIALLY TAXIED WITHOUT ANY PROBLEMS, BUT PULLING ONTO TAXIWAY “F” THE STUDENT STATED THAT THE RUDDER CONTROLS STARTED TO FEEL WEIRD, AND THE STUDENT RESPONDED BY PULLING THE THROTTLE BACK TO IDLE. GROUND SPEED INDICATED ROUGHLY 10 KNOTS AT THE TIME OF THROTTLE IDLING, AND SUBSEQUENTLY STARTED REDUCING. IMMEDIATELY AFTER THE AIRPLANE STARTED PULLING TO THE LEFT, AND THE STUDENT QUICKLY INFORMED INSTRUCTOR THAT HE WAS LOSING CONTROL OF THE AIRPLANE. INSTRUCTOR TOOK THE CONTROL OF THE AIRPLANE AND APPLIED FULL RIGHT RUDDER, WHILE APPLYING FULL RIGHT BRAKE. INSTRUCTOR STATED RUDDERS AND BRAKES FELT MUSHY, AND YIELDED NO RESPONSE RESULTING IN THE AIRPLANES PULL TO THE LEFT. THE AIRPLANE MOVED AT A SLOW RATE OF SPEED WHILE SLOWLY DRIFTING OFF THE TAXI WAY CENTERLINE. EVENTUALLY THE AIRPLANE CAME TO A STOP SLIGHTLY OFF THE TAXI WAY, AT WHICH TIME GROUND CONTROL WAS NOTIFIED.
AFTER THE AIRPLANE HAD COME TO A COMPLETE STOP, THE INSTRUCTOR PROCEEDED TO PUMP ON THE BRAKE PEDAL, AND AFTER A FEW PUMPS HE FELT THE PEDAL STARTING TO RESIST AND THE RIGHT BRAKE CAME BACK TO LIFE. THIS ALLOWED HIM TO REALIGN THE NOSE TOWARDS THE TAXI WAY CENTERLINE, BUT WITH SAFETY IN MIND THE DECISION WAS MADE TO GET THE AIRPLANE TOWED BACK TO THE RAMP, RATHER THAN ATTEMPTING TAXIING BACK.
IN CONCLUSION THEY INADVERTENTLY LOST CONTROL OF THE AIRPLANE DUE TO A TEMPORARY LOSS OF RIGHT BRAKE EFFECTIVENESS. NO PROPERTY, PERSON OR AIRPLANE WAS DAMAGED DURING THIS EVENT.