What was found
IN ROUTE OVER KCOU WHEN THE MASTER CAUTION CAME ON AND THE LT BLEED FAIL LIGHT ILLUMINATED. THE PILOTS FOLLOWED THE CHKLIST & CLOSED LT BLEED VALVE. THE CHKLIST INSTRUCTED THEM TO DESCEND BELOW 14K. ASKED ATC, CENTER WAS VERY BUSY & TO GIVE THEM PRIORITY HANDLING, DECLARED AN EMERGENCY FOR PILOTS. LANDED WITHOUT INCIDENT BACK AT KSUS. MX TEAM FOUND A SMALL RUPTURE IN EVA TUBING. THE THEORY IS THAT IF THE BLEED LINE BREAKS IT WILL MELT THE PLASTIC TUBE AND TURN ON THE LIGHT SO THE CREW CAN TURN OFF THAT BLEED SYSTEM. THE BREAK WAS IN LT L/E JUST INBD OF NACELLE. IT WAS REPAIRED IAW MM 26-11-00. GROUND RUNS WERE PERFORMED AND THE SYS CHECKED, NO DEFECTS NOTED.