STUDENT AND INSTRUCTOR ABOARD C152 ARE UNABLE TO KEEP THE ACFT ON THE RWY WHEN THE LEFT BRAKE FAILS TO RESPOND.

Narrative

I WAS DOING A FULL STOP LNDG WITH MY STUDENT AFTER AN HR LONG LESSON FOR TOUCH AND GOES. WE HAD REDUCED THE THROTTLE TO IDLE AND PRESSED ON THE BRAKES. THE BRAKES WERE NOT RESPONDING AS WELL AS THEY SHOULD HAVE. AS IT SAYS ABOVE; I HAVE 280 HRS IN TYPE AND I COULD FEEL THAT THE ACFT WAS NOT DOING WHAT IT WAS DESIGNED TO DO (IE; STOP); BOTH MY STUDENT AND I APPLIED FULL BRAKE PRESSURE AND THE ACFT BEGAN TO YAW TO THE R INDICATING A MALFUNCTIONING L BRAKE. I ATTEMPTED TO APPLY FULL L RUDDER AND BRAKE BUT THE COMBINED EFFECT OF MOMENTUM AND THE FAULTY L BRAKE; THE AIRPLANE SKIDDED ONTO THE GRASS AND CLIPPED A TXWY AND RWY LIGHT. MY STUDENT AND I BOTH PULLED THE MIXTURE TO IDLE CUTOFF AND STOPPED THE PLANE; THANKFULLY; ON A TXWY. WE THEN PUSHED THE PLANE CLR OF THE HOLD SHORT LINE AND AWAY FROM THE RWY. IN RETROSPECT; I COULD HAVE PULLED THE MIXTURE EARLIER AND PROBABLY SLOWED THE PLANE DOWN ON THE RWY BUT IT WAS SUCH A SHOCK AT THE RATE THE PLANE YAWED TO THE R THAT WE/I DIDNT' HAVE ENOUGH TIME TO REDUCE IT. I ALSO WANTED TO CLR THE ACTIVE RWY AS THERE WAS AN ACFT ON FINAL THUS THE THOUGHT TO STOP ON THE RWY DIDN'T CROSS MY MIND UNTIL I KNEW WE WERE SKIDDING OFF THE RWY. THE MECHS INSPECTED THE ACFT AFTERWARD AND FOUND NO VISIBLE DAMAGE HOWEVER; ANOTHER CFI RPTED OF SIMILAR IRREGULARITIES WITH THE PLANE ON THE PREVIOUS DAY. I THINK A FURTHER INSPECTION OF THE ACFT IS REQUIRED AS WELL AS A THOROUGH FLT TEST BEFORE PUTTING IT BACK INTO CIRCULATION.