PLT OF AN ATX CARGO TWIN TURBOPROP LOST DIRECTIONAL CTL OF THE ACFT DURING LNDG DUE TO THE L BRAKE STICKING AND RAN OFF THE SIDE OF THE RWY. OPPOSITE BRAKING OR REVERSE THRUST DID NOT PREVENT THE ACFT FROM VEERING TO THE L AND OFF THE RWY. THE RPTR NOTICED THE ACFT PULL TO THE L DURING TKOF AFTER MANUALLY HOLDING BRAKES PRIOR TO TKOF.

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FOR A REASON THAT IS YET TO BE DETERMINED; THE L BRAKES WERE LOCKED DURING TOUCHDOWN ON RWY 21 AT TTF ARPT. THIS CAUSED AN UNCOMMANDED VEER TO THE L. MY ONLY THOUGHT WAS TO KEEP THE ACFT ON THE RWY. I COULD FEEL A PULL OR DRAG FROM THE L; BUT AT THE TIME I COULDN'T IMAGINE THE CAUSE. MY ACTION WAS IMMEDIATE FULL R RUDDER AND BRAKING ACTION; FOLLOWED CLOSELY BY FULL R BETA. THESE ACTIONS WERE NOT ENOUGH AND THE ACFT VEERED OFF THE RWY. AN EVENT PRIOR TO THIS OCCURRED IN YIP THAT IN RETROSPECT APPEARS TO BE A HINT THAT A PROB EXISTED. AT YIP (WHICH IS A 10 MIN FLT FROM TTF) I WAS COMMANDED BY TWR TO TAXI INTO POS AND HOLD. WHILE WAITING FOR TKOF CLRNC; I HELD THE BRAKES MANUALLY. AS TKOF ROLL STARTED ALL SEEMED NORMAL. HOWEVER; AT ABOUT 60 KTS THE ACFT BEGAN TO PULL L. I WAS ABLE TO TAKE OFF PRIOR TO ANY INCIDENT; BUT OBVIOUSLY THE EVENTS ARE RELATED. I DON'T KNOW IF I COULD HAVE DONE ANYTHING DIFFERENT; BUT I THINK I SHOULD HAVE CONSIDERED MY NEXT LNDG AND PERHAPS HAVE BEEN EXPECTING A DIFFICULT LNDG.