Narrative:

The problem: during taxi back to the hangar; we lost brake pressure in our left brake causing a runway incursion (breaching a runway hold short line and hitting a taxi light). We had just landed and taxied to hold short of runway xxl. We made a right turn to hold short of the runway and the aircraft did not stop before the hold short line. My student; a very experienced pilot; recently purchased a cessna 172. He had it fixed up at a repair shop and refurbished. I had flown this aircraft specifically for about 30 hours in training; and have over 300 hours of total time in cessna 172s. What happened is we lost brake pressure in our left brake while taxiing back to the hangars. When we started the flight we both did a brake test (I was sitting right seat) and found the brakes to work perfectly fine. We went out and had a nice flight about 30 miles south of the airport; came back and did 3 touch and go's; and made a full stop landing on #3. Upon touching down; we exited runway xxl; and taxied up to hold short of runway xxr. The student turned right and attempted to stop prior to the hold short line. Braking did not occur in time. I started saying in a reminding voice 'hold short of xxr; we need to hold short here...etc.' when I realized we were going to cross the hold short line; I applied full brake pressure. During the right turn to hold short; I noticed we were taxiing further towards the grass as well; so when I applied pressure I applied left brake pressure harder to try and turn us the other way. We ended up going into the grass past the hold short line and hit a taxi light with the prop. Airport authority shut down runway xxr; and we got a tug to tow us off the runway to maintenance.what I learned: my student was trying to stop - and when I slammed on the brake pressure (not knowing at the time we lost left brake) we ended up just going further to the right. This caused us to go into the grass and hit a light. We had already crossed the hold short line at this point; but my actions did not help in slamming on the brakes at this point. I think the best course of action to solve this problem would have been to pull the mixture to idle; hopefully starving the engine of fuel and stopping the prop before it hit a light. I should have realized that maybe it wasn't a problem with the student not wanting to stop; but a maintenance problem. My initial instinct was to slam on the brakes; but moving forward I will try to reevaluate my decisions before acting. Student: a gentleman who has been flying for longer than I have been alive twice over. He did not have a current medical; but was working on it. He wanted to fly with a CFI for the purpose of flight safety; in case something popped up.

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Original NASA ASRS Text

Title: C172 flight instructor and certified student reported a left brake failure during taxi which resulted in a runway incursion and taxiway excursion. The propeller struck two taxiway lights.

Narrative: The problem: during taxi back to the hangar; we lost brake pressure in our left brake causing a runway incursion (breaching a runway hold short line and hitting a taxi light). We had just landed and taxied to hold short of runway XXL. We made a right turn to hold short of the runway and the aircraft did not stop before the hold short line. My student; a very experienced pilot; recently purchased a Cessna 172. He had it fixed up at a repair shop and refurbished. I had flown this aircraft specifically for about 30 hours in training; and have over 300 hours of total time in Cessna 172s. What happened is we lost brake pressure in our left brake while taxiing back to the hangars. When we started the flight we both did a brake test (I was sitting right seat) and found the brakes to work perfectly fine. We went out and had a nice flight about 30 miles south of the airport; came back and did 3 touch and go's; and made a full stop landing on #3. Upon touching down; we exited runway XXL; and taxied up to hold short of runway XXR. The student turned right and attempted to stop prior to the hold short line. Braking did not occur in time. I started saying in a reminding voice 'hold short of XXR; we need to hold short here...etc.' When I realized we were going to cross the hold short line; I applied full brake pressure. During the right turn to hold short; I noticed we were taxiing further towards the grass as well; so when I applied pressure I applied left brake pressure harder to try and turn us the other way. We ended up going into the grass past the hold short line and hit a taxi light with the prop. Airport authority shut down runway XXR; and we got a tug to tow us off the runway to maintenance.What I learned: My student was trying to stop - and when I slammed on the brake pressure (not knowing at the time we lost left brake) we ended up just going further to the right. This caused us to go into the grass and hit a light. We had already crossed the hold short line at this point; but my actions did not help in slamming on the brakes at this point. I think the best course of action to solve this problem would have been to pull the mixture to idle; hopefully starving the engine of fuel and stopping the prop before it hit a light. I should have realized that maybe it wasn't a problem with the student not wanting to stop; but a maintenance problem. My initial instinct was to slam on the brakes; but moving forward I will try to reevaluate my decisions before acting. Student: a gentleman who has been flying for longer than I have been alive twice over. He did not have a current medical; but was working on it. He wanted to fly with a CFI for the purpose of flight safety; in case something popped up.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.