A CARGO PART 135 C208 PIC LOSES CTL OF THE ACFT DURING TAXI AND DEPARTS THE TXWY AT ADS; TX.

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THERE WAS A MINOR INCIDENT INVOLVING A CARAVAN AND MYSELF DUE TO A COMPLETE LOSS OF BRAKING PWR ON TXWY U AT ADDISON ARPT IN DALLAS; TX. I SHOWED UP TO THE ARPT EARLIER FOR A SCHEDULED DEP AND NOTICED THE ACFT WAS IN THE HANGAR BEING WORKED ON BY MAINT. MAINT BEGAN TO PUT THE ACFT BACK TOGETHER AND TOWED THE PLANE OUT OF THE HANGAR TO CONDUCT RUN-UP CHKS ON THE MAINT PREVIOUSLY PERFORMED. THEY HAD A FEW MINOR ADJUSTMENTS AND RELEASED THE ACFT TO ME AND ASKED ME TO CHK THE BATTERY VOLTAGE OUTPUT ON START-UP AND THE SUCTION GAUGE AT 1750 PROP RPM. ANOTHER FEW ADJUSTMENTS WERE MADE AND FO AND I WERE RELEASED. THE FLT BLOCKED OUT AND WE BEGAN TO TAXI OUT OF THE RAMP. I TAPPED THE BRAKES AND NOTICED THEY WERE A LITTLE BIT MUSHY. I IMMEDIATELY TURNED AND TOLD FO 'THESE BRAKES FEEL KIND OF SPONGY.' FO DIDN'T THINK MUCH OF IT AT THAT MOMENT BUT UNBEKNOWNST TO US; BRAND NEW BRAKES HAD JUST BEEN PUT ON THE ACFT. AS I BEGAN TO TAXI OUT OF THE RAMP AND ONTO TXWY U; I BEGAN TO CONDUCT THE ROUTINE TAXI CHKS WHILE STEERING THE ACFT WITH MY FEET AND PWRING IN THE IDLE POS SO AS NOT TO TAXI FAST ON THE NARROW TXWY. AS I APCHED TXWY A; THE ACFT GOT OFF CTRLINE APPROX 10 DEGS FROM TXWY U. I NEEDED TO SLOW DOWN TO MAKE A SMOOTH R TURN ONTO TXWY A AND SIMULTANEOUSLY CORRECT L; BACK TO CTRLINE; SO I SIMPLY APPLIED A LITTLE BIT OF BRAKING WHILE STEERING SLIGHTLY TO THE L. THE L MAIN GEAR BRAKE DID NOT RESPOND; BUT THE R MAIN GEAR DID; CANCELING MY SLIGHT L STEERING CORRECTION FROM THE APPROX 10 DEG CTRLINE DEV. WHEN I REALIZED THAT THE ACFT WAS NOT CORRECTING BACK TO THE L; I APPLIED MORE L BRAKE AND A LITTLE R BRAKE; BUT THE ACFT BEGAN TO TURN MORE TO THE R AND FURTHER FROM THE CTRLINE. IT SEEMED LIKE BOTH BRAKES WERE WEAK; BUT THE R BRAKE SEEMED SLIGHTLY MORE EFFECTIVE. WHEN THE DEV SEEMED LIKE IT WAS GETTING OUT OF HAND; I APPLIED FULL L AND R BRAKING PWR AND FULL THRUST REVERSE AND TOLD FO THAT WE HAD LOST THE BRAKES. I PUT ALL MY BODY WT ON BOTH BRAKES; BUT THE ACFT KEPT ROLLING WITH A SLIGHT R TURN. THE L BRAKE HAD ZERO RESPONSE AND THE R BRAKE BARELY RESPONDED. FO TOLD ME; 'I HAVE THE PLANE!' HE; TOO; APPLIED FULL BRAKES PRIOR TO TOUCHING THE GRASS ON THE R SIDE OF THE TXWY; BUT THE ACFT SIMPLY DID NOT RESPOND EFFECTIVELY TO OUR RECOVERY INPUTS. AS THE ACFT'S R MAIN GEAR CAME OFF THE TXWY AND ONTO THE GRASS; THE R MAIN GEAR BRAKE LOCKED AND THE PLANE TURNED SHARPLY TO THE R MAKING A 180 DEG TURN INTO THE GRASS. AS THE ACFT CAME TO A COMPLETE STOP IN THE GRASS; I IMMEDIATELY CALLED GND CTL AND TOLD THEM THAT WE HAD TO STOP TO CHK SOMETHING. GND CTL RESPONDED BY SAYING; 'LET ME KNOW WHEN YOU'RE READY TO TAXI TO RWY 15.' FO CALLED DISPATCH AND REQUESTED MAINT TO COME AND PULL US OUT OF THE GRASS WITH THE TUG. WHILE BOTH FO AND I WAITED FOR THE TUG; WE SHUT DOWN THE ACFT AND BOTH AGREED THAT THE BRAKES HAD FAILED. WHEN MAINT SHOWED UP; THEY ASKED US WHAT HAD HAPPENED; AND I RESPONDED THAT WE HAD LOST BRAKING PWR. THE MECH TOLD ME RIGHT THEN THAT THE ACFT HAD JUST HAD NEW BRAKES PUT ON AND THAT THERE WAS POSSIBLY STILL AIR IN THE HYD LINE. HE ASKED ME TO PUMP THE BRAKES. I BEGAN TO PUMP THE BRAKES AND THE PRESSURE BEGAN TO INCREASE WITH THE PUMPING. THE MECHS TOWED US OUT OF THE GRASS AND ONTO A WIDE RAMP AREA WHERE I TESTED THE BRAKES AND THINGS GOT MORE CTLABLE. THE MECH TOLD US TO BREAK IN THE BRAKES BY APPLYING THEM WHILE WE TAXI. FO AND I COMPLETED THE BRAKE CHK AND THE BRAKES WERE MORE CTLABLE; SO WE CONTINUED WITH OUR MISSION AND PROCEEDED TO TAXI WITHOUT FURTHER INCIDENT OR ANY DAMAGE TO PROPERTY. I BELIEVE THIS SIT WAS CAUSED BY MAINT NOT CONDUCTING A POST TEST RUN OF THE ACFT BRAKES AND NOT COMMUNICATING TO ME; THE PLT; THAT BRAKES WERE RECENTLY REPLACED. THIS SIT CAN BE AVOIDED IN THE FUTURE BY COMMUNICATING AS CLOSE AS POSSIBLE WITH MECHS AFTER ANY AND ALL MAINT INSPECTIONS.