C172 instructor pilot reported loss of directional control after landing when the left brake failed.

Narrative:

I was refreshing a post-solo student who had taken some time off from his instruction in traffic pattern work. We were flying a C172M that had recently returned from having a new windscreen and cowling installed. We performed a thorough preflight check and; while taxiing to the runway in use; checked the brakes for proper operation. The winds were calm when we first took off; but I advised the student that the forecast was for winds to pick-up from the north and we either might get a change of runway during our flight or we might get some tailwinds on final.we did some touch and goes; one go-around; and on our last approach; as expected; we got a tailwind which caused our last landing to go a little long (but not unreasonably so). During the flare; I told the student to make this a full stop/taxi back instead of a touch and go. The flare and landing was good; but the aircraft started to veer off to the right. I told the student to add more left rudder to stay on the centerline and he looked kind of surprised. We were now drifting to the right side of the runway and the end of the runway. I told him; 'my aircraft' and found that the left brake had failed; the pedal was all the way to the fire wall. There is a grass boundary between [the two runways]. I chose to maneuver the plane onto the grass strip off to the right as opposed to hitting the runway threshold lights.we came to a stop. I advised tower that our left brake had failed. Tower asked if we needed assistance. I reported that everything seemed fine and they had us taxi back to our flight school. After a couple of pumps; I was able to get some braking action on the left brake. Ops came by and I explained what happened. He said that he had inspected the area where we stopped and could see no property (i.e. Lights) damage. The mechanic at the flight school found that the caliper had come loose. He was able to repair the brake in 30 minutes.the mechanic said that if it were to happen again; one can usually pump up the brake with 2-3 pumps of the pedal. He also realized that I had to take control of the aircraft; assess what was going on; and make a decision based on how much runway remained. And there could be the possibility of a brake cylinder failure; which wouldn't have been helped by the pumping.a teachable moment to troubleshoot a problem and make a safe decision based on a couple of options. And it was all on the ground.

Original NASA ASRS Text

Title: C172 instructor pilot reported loss of directional control after landing when the left brake failed.

Narrative: I was refreshing a post-solo student who had taken some time off from his instruction in traffic pattern work. We were flying a C172M that had recently returned from having a new windscreen and cowling installed. We performed a thorough preflight check and; while taxiing to the runway in use; checked the brakes for proper operation. The winds were calm when we first took off; but I advised the student that the forecast was for winds to pick-up from the north and we either might get a change of runway during our flight or we might get some tailwinds on final.We did some touch and goes; one go-around; and on our last approach; as expected; we got a tailwind which caused our last landing to go a little long (but not unreasonably so). During the flare; I told the student to make this a full stop/taxi back instead of a touch and go. The flare and landing was good; but the aircraft started to veer off to the right. I told the student to add more left rudder to stay on the centerline and he looked kind of surprised. We were now drifting to the right side of the runway and the end of the runway. I told him; 'My aircraft' and found that the left brake had failed; the pedal was all the way to the fire wall. There is a grass boundary between [the two runways]. I chose to maneuver the plane onto the grass strip off to the right as opposed to hitting the runway threshold lights.We came to a stop. I advised tower that our left brake had failed. Tower asked if we needed assistance. I reported that everything seemed fine and they had us taxi back to our flight school. After a couple of pumps; I was able to get some braking action on the left brake. Ops came by and I explained what happened. He said that he had inspected the area where we stopped and could see no property (i.e. lights) damage. The mechanic at the flight school found that the caliper had come loose. He was able to repair the brake in 30 minutes.The mechanic said that if it were to happen again; one can usually pump up the brake with 2-3 pumps of the pedal. He also realized that I had to take control of the aircraft; assess what was going on; and make a decision based on how much runway remained. And there could be the possibility of a brake cylinder failure; which wouldn't have been helped by the pumping.A teachable moment to troubleshoot a problem and make a safe decision based on a couple of options. And it was all on the ground.

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.

Attributes
ACN	1330581
Time
Date	201602
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Landing
Route In Use	None
Flight Plan	None
Component
Aircraft Component	Normal Brake System
Person 1
Function	Instructor
Qualification	Flight Crew Commercial Flight Crew Flight Instructor Flight Crew Multiengine Flight Crew Instrument
Experience	Flight Crew Last 90 Days 55 Flight Crew Total 4100 Flight Crew Type 3100
Events
Anomaly	Aircraft Equipment Problem Less Severe Ground Event / Encounter Loss Of Aircraft Control Ground Excursion Runway