What was found
CLIMBING TO 10,000 FT, LT BLEED AIR DUCT FAIL ANNUNCIATOR HAD ILLUMINATED. ENG THRUST REDUCED, AND CABIN PRESSURIZATION SOURCE TURNED OFF. TWO MINS LATER COCKPIT & CABIN HAD FILLED WITH SMOKE FOLLOWED BY EMERGENCY PRESSURIZATION VALVE OPENING. CREW DONNED OXYGEN MASKS & GOGGLES AS SMOKE HAD BECOME FAIRLY THICK. AN EMERGENCY WAS DECLARED & AIRCRAFT MADE AN UNEVENTFUL LANDING. MX FOUND A Y-DUCT ON LEFT SIDE TO BE MISALIGNED AND LEAKING. A NEW SEAL WAS INSTALLED ON THE OUTBOARD DUCT AT THE TOP OF THE FUSELAGE ABOVE THE PYLONS. THE EMERGENCY PRESSURIZATION VALVE WAS FOUND TO BE STUCK IN THE OPEN POSITION. A NEW VALVE WAS INSTALLED. OPERATIONAL CHECKS WERE NORMAL. AIRCRAFT WAS RETURNED TO SERVICE.