AFTER A HYDRAULIC SYS MALFUNCTION, THE CAPT RETURNED TO ARPT TO EXCHANGE ACFT. COMPANY PLT HAD COMPLETED DAY OF FLYING AND WAS ASKED TO TAXI ACFT TO MAINT HANGAR. DEPARTING CAPT WAS IN PROCESS OF MANUALLY PUMPING HYD SYS TO SUFFICIENT PRESSURE TO ENSURE BRAKING ACTION. REPOSITIONING CAPT TOOK LEFT SEAT AND BEGAN TAXI TO HANGAR, CONFIRMED RAMP AREA REQUIRED POWER AND BRAKING TO MANEUVER AWAY FROM BOEING 767 AND FUEL TRUCK. PLT LOST BRAKE PRESSURE AND PLACED PROPS IN REVERSE TO PREVENT COLLISION WITH TRUCK. COLLISION WAS IMMINENT SO PLT ELECTED TO SHUT DOWN ENGINES. AS PROPS TRANSITIONED FROM POWERED REVERSE A BURST OF THRUST ACCELERATED ACFT TO IMPACT WITH FUEL TRUCK AND DELTA JET. EXAM DETERMINED INTERNAL HYDRO PUMP FAILED DRIVE SHAFT DUE TO IMPROPERLY SECURED RETAINER NUT. THE COMPANY HAS SINCE MADE IT MANDATORY FOR 2 PLTS TO BE ON A TAXIIED ACFT.