What was found
LAS - AIR TURNBACK - 2/22/01 - ALL LAVS WOULD NOT FLUSH (CIDS PANEL RED LIGHT WAS ON, WASTE AT 0 PERCENT FULL, WATER AT 40 PERCENT FULL); AIRCRAFT DESTINATION WAS JFK SO THE FLIGHT RETURNED TO LAS. THE LAV SYSTEM HAD PREVIOUSLY BEEN PLACED ON MEL 38-30-01 FOR THE VAC GENERATOR INOP. HOWEVER THE SYSTEM FAILED TO FLUSH USING DIFFERENT DIFFERENTIAL PRESSURE AT ALTITUDE. IN LAS MAINTENANCE FOUND THE WASTE LINE CLOGGED AT THE LAV WASTE TANK; THE LINE WAS DISCONNECTED AND CLEANED OUT; AFTER CLEANING THE SYSTEM FUNCTIONALLY CHECKED GOOD.