B767-300 CREW HAD 4 OF 8 ACFT TOILETS BECOME INOP WITH A FULL PAX LOAD; PRIOR TO ENTERING ATLANTIC OCEANIC AIRSPACE FOR A FLT FROM THE UNITED STATES TO EUROPE.

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AIR ABORT AND OVERWT LNDG. THIS RPT ADDRESSES THE AIR ABORT OF A FLT SCHEDULED TO ZRH. OPS WERE NORMAL UNTIL ENRTE APPROX N OF HALIFAX; NOVA SCOTIA. I WAS NOTIFIED BY THE CABIN CREW THAT WE HAD IN -- FAIRLY RAPID SUCCESSION -- LOST THE FUNCTIONING OF 4 LAVATORIES OUT OF A TOTAL OF 8. THEY WOULD NOT FLUSH AND HAD BECOME QUITE FULL. I CONTACTED THE RELIEF PLT ON BREAK AT THE TIME AND ASKED HIM TO ASSESS THE PROB. FO AND I CONSULTED OUR ONBOARD RESOURCES AND INITIATED CONTACT WITH ACR MAINT CTLR THROUGH DISPATCH IN ORDER TO GET ADDITIONAL INFO RELEVANT TO THE SYS MALFUNCTION. THE ACR MAINT CTLR REPRESENTATIVE INDICATED THAT THE 4 LAVATORIES ARE ALL ASSOCIATED WITH A SINGLE SYS AND THE OTHER 4 ARE ASSOCIATED WITH A SEPARATE SYS. MAINT SUGGESTED THE TIME HONORED TECHNIQUE OF ATTEMPTING 1 RESET OF THE MALFUNCTIONING SYS COUPLED WITH FLUSHING SOME ICE THROUGH AN AFT LAVATORY. WE ATTEMPTED THIS PROC WITHOUT SUCCESS. I REQUESTED INFO FROM MAINT REPRESENTATIVE AS TO WHETHER THE REMAINING LAVATORY SYS HAD ADEQUATE CAPACITY FOR THE TRANS-ATLANTIC XING; GIVEN THE FULL PAX LOAD OF 161 PAX PLUS CREW. I WAS INFORMED THAT CAPACITY SHOULD BE ADEQUATE ASSUMING THAT THE LAVATORY TANKS HAD BEEN FULLY EMPTIED PRIOR TO DEP AND BARRING FURTHER FAILURE OF THE LAVATORY SYS. SHORTLY THEREAFTER; I WAS INFORMED THAT WE HAD EXPERIENCED AN ADDITIONAL FAILURE OF 1 LAVATORY ASSOCIATED WITH THE REMAINING SYS. NATURALLY; THIS SEEMED SOMEWHAT MORE THAN COINCIDENCE AND STRONGLY SUGGESTED THAT THE LAVATORIES HAD NOT BEEN PROPERLY SVCED OR PERHAPS THAT WE WERE EXPERIENCING SOME SIGNIFICANT PROGRESSIVE SYS FAILURE. AFTER CONSULTATION WITH THE CREW AND DISPATCH; I ELECTED TO MAKE AN INTERMEDIATE STOP IN ORDER TO RECTIFY THE SYS PROB. THE COURSE CHANGE WAS MADE PRIOR TO ENTERING THE NORTH ATLANTIC RTE STRUCTURE SO AS TO AVOID THE ADDITIONAL HAZARDS ASSOCIATED WITH A COURSE REVERSAL ONCE IN THE TRACK STRUCTURE. DISPATCH ELECTED NOT TO ATTEMPT A SVCING AT HALIFAX; BUT INSTEAD TO RETURN US TO JFK WHERE AN ACFT CHANGE WOULD BE PLANNED. THE RETURN TO JFK WAS UNEVENTFUL AND; AFTER ADDITIONAL CONSULTATION WITH MAINT; AN OVERWT LNDG WAS ACCOMPLISHED. ALL OVERWT LNDG CONSIDERATIONS WERE ADDRESSED AND THE TOUCHDOWN WAS SMOOTH AT 300 FPM AND 319400 LBS. IN MY JUDGEMENT; LNDG THIS SMALL AMOUNT OVERWT WAS AS SAFE AS THE ALTERNATIVE OF HOLDING TO BURN FUEL. ALSO; AN EXPEDITIOUS LNDG COULD REDUCE EXPOSURE TO THESE LAVATORY FAILURES OF UNKNOWN NATURE OR ADDITIONAL PROBS. AFTER LNDG; INITIAL MAINT INVESTIGATION INDICATED THAT LAVATORY SVCING HAD NOT BEEN SUCCESSFULLY ACCOMPLISHED DUE TO A SYS COMPONENT FAILURE. ADDITIONALLY; THERE WAS EVIDENCE THAT THE LAVATORY SYS WAS LEAKING. THE ACFT WAS TAKEN OTS.