HOLDING SHORT OF RWY FOR TKOF PLT ALLEGEDLY LOST BRAKES AND CROSSED HOLD LINE. RETURNED TO RAMP AND FOUND NO PROB WITH BRAKES.

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ACFT IS A CESSNA 172. WE WERE TAXIING AT IAD OUT TO RWY 30. I LOST MY R BRAKE JUST SHORT OF THE W2 INTXN. I TOLD GND THAT I LOST MY R BRAKE AND IN AN ATTEMPT TO STOP THE AIRPLANE COULD ONLY MAKE L TURNS. THE GND CTLR UNDERSTOOD AND PERMITTED A L 360 DEG TURN WHICH PUT ME AT THE W2 INTXN OF RWY 30. AT THAT POINT; MY BRAKES FAILED TOTALLY AND MY ACFT SLOWLY CREEPED FORWARD DUE TO THE TORQUE OF THE ENG. I THEN UNKNOWINGLY PASSED THE HOLD SHORT LINE FOR RWY 30 BY ABOUT 20 FT. 2 ACR PLANES DEPARTED AND AT THAT POINT I JUST COULD NOT HOLD THE PLANE STILL. DULLES TWR CLRED ME FOR TKOF. I ROLLED OUT ON THE RWY AND THEN DECIDED TO ABORT AND GO BACK AND LOOK AT THE BRAKES. GND THEN CLRED ME TO TAXI WITH 'TOTAL BRAKE FAILURE' BACK TO THE FBO. ON THE WAY BACK; THE BRAKES REGAINED FUNCTION. UPON INSPECTION; THERE SEEMED TO BE NO PROBS WITH THE BRAKE SYS. NOBODY QUERIED MY POS AT W2 INTXN OF RWY 30. MY PLANE WAS IN NO DANGER AND NEITHER WAS ANY OTHER DEPARTING TFC. MY ONLY CONCERN WAS THAT I DID PASS THE HOLD SHORT BY APPROX 20 FT AND THEN RESTED. WHEN MY PLANE BEGAN TO ROLL AGAIN; I WAS CLRED ONTO THE RWY. THE HOLD SHORT LINE AT THE W2 INTXN OF RWY 30 IS VERY VAGUE. THE POINT IS WORN AND VERY LIGHT. THIS SIT MAY HAVE BEEN PREVENTED IF THE LINES WERE MORE DISTINCT.