B757 Captain reported diverting due to a lavatory failure and was advised by a Maintenance Technician that the lavatory had a cracked fill line and liquid was discovered in the E&E compartment.

Narrative

Approximately 30 minutes after departure; the flight attendants (FAs) called and advised that the first class lavatory had stopped flushing. The lavatory had been working properly on the ground during an approximate 2 1/2 hour delay due to maintenance issues with passengers on board. We advised the FAs to try and pour 3 coffee pots of water into the lavatory and see if it would flush correctly. That attempted resolution to fix the problem was unsuccessful.A SAT call was placed to Dispatch and Maintenance Control was conferenced into the call. The situation was explained to them; and Maintenance Control recommended pouring an additional three coffee pots full of water into the lavatory and attempt flushing. The FA accomplished this which still provided no resolution to the issue.After reviewing the MEL used to defer a lavatory; we requested that Maintenance Control perform a lavatory calculation to determine if we had enough capacity with the remaining 3 systems to complete the flight to ZZZ based on 9 hours of flight time (2 hours of usage during the departure delay and 7 hours enroute). We also asked the Dispatcher to talk to Operations for their guidance; as they would need to approve a subsequent departure if deferred per the MEL. Operations was okay with continuation; but Maintenance Control advised that calculated lavatory capacity was only 2 gallons remaining at landing among the 3 remaining lavatories. After discussion with the FO; we determined that there was not enough margin to confidently complete the flight with adequate lavatory capacity. The decision was made to land at ZZZ1.Dispatch and ATC were advised of the plan. No emergency was declared and no additional services were used. The overweight landing checklist was completed; and an uneventful landing was accomplished. Electronic Log Book (ELB) write-ups were completed for the lavatory issue and overweight landing. A thorough crew debrief was accomplished after arrival at the gate. A Maintenance Technician did advise us before we left the airplane that they had discovered a cracked fill line for the lavatory; and that liquid was discovered in the E&E compartment. He stated the aircraft was being removed from service. The Chief Pilot was contacted and debriefed as required by the FOM; and a report was filed. Passengers were moved to a different aircraft and continued on to ZZZ.