LR23 CROSSED RWY HOLD LINE AT CNO BECAUSE OF FAULTY PARKING BRAKE.

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ON APR/THU/04 AT CHINO ARPT; CA; AS PIC OF A LEAR JET 25 AND MR X ACTING AS FO; WE TAXIED TO AND HELD SHORT OF RWY 26L ON TXWY G PER GND CTL INSTRUCTION. AFTER HAVING SET THE ACFT PARKING BRAKE ON; I LEFT THE COCKPIT AND ENTERED THE CABIN AREA TO READJUST THE CABIN DOOR MECHANISM DUE TO A DOOR LIGHT ILLUMINATION ON THE ANNUNCIATOR PANEL DURING TAXI. WHEN I WAS IN THE CABIN AREA; THE FO WAS LOADING UP THE GPS FOR THE DAY'S FLT. DURING THIS TIME; I NOTICED THE ACFT BEGIN TO ROLL WHILE THE PARKING BRAKE WAS SET ON AND TOLD THE FO TO APPLY THE BRAKES; WHICH HE DID; AND STOPPED THE ACFT SHORT OF THE RWY. THE ACFT DID CROSS THE HOLD LINE; BUT NEVER ENTERED THE RWY. THE ACFT'S MECHS WERE NOTIFIED OF THE ACFT'S PARKING BRAKE FAILURE AND REPAIRED PER THE ACFT MANUFACTURER MAINT SPECS AND ENTERED IN THE ACFT LOGBOOK. NO OTHER ACFT WERE INVOLVED. SUPPLEMENTAL INFO FROM ACN 614167: WHEN I WAS PLACING THE WAYPOINTS INTO THE GPS; THE CAPT TOLD ME TO APPLY THE BRAKES BECAUSE THE AIRPLANE HAD BEGUN TO ROLL WHILE THE PARKING BRAKE WAS ON. I STOPPED THE LEAR SHORT OF THE RWY; THOUGH IT HAD CROSSED THE HOLD LINE. NO OTHER AIRPLANES WERE INVOLVED.