What was found
FLIGHT DIVERTED DUE TO ALL LAVATORIES GOING INOP. HISSING SOUND NOTED BELOW 1R. TRIED TROUBLESHOOTING TO NO AVAIL. [SUP 06-06-07] EXISTING SERVICE BULLETIN 747-38-2109 DIRECTS TO REPLACE RELAY R8432 (LEVEL SENSING TRANSFER RELAY). THIS RELAY IS IDENTIFIED AS THE SINGLE POINT FAILURE WHICH CAUSES ALL 4 WASTE TANK CONTROLLERS TO LOSE PWR. REPLACED RELAY. A/C RETURNED TO SERVICE WITH NO REPEAT OF PROBLEM.