Pilot reported a main wheel brake locked up on taxi; causing a taxiway excursion.

Attributes
ACN	1727020
Time
Date	202002
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	Taxi
Route In Use	None
Component
Aircraft Component	Normal Brake System
Person 1
Function	Instructor
Qualification	Flight Crew Commercial Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 112 Flight Crew Total 440 Flight Crew Type 127
Events
Anomaly	Aircraft Equipment Problem Less Severe Ground Event / Encounter Loss Of Aircraft Control Ground Excursion Taxiway

Narrative:

The aircraft to be used for training was previously squawked for a front nose wheel shimmy. Maintenance at our flight school looked it over as well as the main wheel brakes prior to my scheduled flight and said that the aircraft was safe to fly. My student and I followed the correct procedures and checklists for engine-start and taxi. Both the student and I checked the brakes prior to taxi and found them functioning properly. As we were on our taxi for departure and while making a right-turn (at a safe speed) onto D from B; our left brake locked and caused our aircraft to exit the taxiway just east of [taxiway] pointing toward the active runway. I immediately reduced power and pulled the mixture to I.C.O to stop our aircraft. I reported our deviation to ground control and requested assistance and then informed them that I would contact our school. The school's maintenance personnel helped us unlock the brake and we were able to taxi back to the school ramp. Maintenance then investigated the issue in more detail.

Original NASA ASRS Text

Title: Pilot reported a main wheel brake locked up on taxi; causing a taxiway excursion.

Narrative: The aircraft to be used for training was previously squawked for a front nose wheel shimmy. Maintenance at our flight school looked it over as well as the main wheel brakes prior to my scheduled flight and said that the aircraft was safe to fly. My student and I followed the correct procedures and checklists for engine-start and taxi. Both the student and I checked the brakes prior to taxi and found them functioning properly. As we were on our taxi for departure and while making a right-turn (at a safe speed) onto D from B; our left brake locked and caused our aircraft to exit the taxiway just east of [taxiway] pointing toward the active runway. I immediately reduced power and pulled the mixture to I.C.O to stop our aircraft. I reported our deviation to Ground Control and requested assistance and then informed them that I would contact our school. The school's maintenance personnel helped us unlock the brake and we were able to taxi back to the school ramp. Maintenance then investigated the issue in more detail.

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.