What was found
AT CRUISE ALT 43,000 FT PILOT NOTICE CABIN ALT WARNING LIGHT ILLUMINATED. THE PILOT INITIATED A DESCENT TO 27,000 FT PRESSURIZATION STARTED WORKING NORMAL. FLT CONTINUED AT 35,000 FT & WORKED NORMAL REST OF FLT. AFTER TROUBLESHOOTING SYS THE WINDSHIELD DEICE SHUTOFF VALVE WAS FOUND LEAKING AT FLANGE OF THE VALVE. VALVE WAS REPLACED & SYS WAS CHECKED FOR OPERATION, SYS CHECKED NORMAL. POSSIBLE FACTORS EFFECTING THE SYS, ACFT WAS IN A HEAVY RAIN STORM PRIOR TO TAKEOFF 3-4 HRS & WAS FLOWN IN HEAVY RAIN FOR ABOUT 45 MIN PRIOR TO OCCURRENCE. A LOT OF MOISTURE IN NOSE AVIONICS UPON LANDING. DRAINS WERE FOUND CLEAR. THE AVIONICS DOOR SEALS INSPECTED, NO DEFECTS NOTED. TIME ON VALVE UNKNOWN, ESTIMATED 5300 HRS.