A Lead Mechanic reports about a B767 that arrived with forward and mid lavatories flooded from a blocked forward water drain line. Item was deferred; but lav flooded again during climb; the aircraft was subsequently returned to the departure field.

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Reporter stated when the B767 had landed; the reversing and braking action of the aircraft had caused a lot of the water in the blocked-up lav sink and Galley drains to flow forward; up and out of the first class and Mid M2 lav sink drains. He did not have any idea of the amount of waste water that still remained in all the lav drain lines that came out at the mid M2 lav sink drain during takeoff; when he had deferred the mid lavs sink drains and shut-off water to the faucets. Reporter stated they had accomplished all the procedures required by the MEL; including placarding the sink and galley drains; shutting-off the water supply to the faucets; having any liquid or fluid placed in the aircraft trash containers and providing handi-wipes out to passengers.Reporter stated that in retrospect; he probably should have taken the B767 out of service when the aircraft first arrived with the original lav flooding write-up. Because that's exactly what maintenance did after the air turnback. Mechanics opened up numerous cabin floorboards and found the lav sink drain lines electric heater tapes that are wrapped around the drain lines; had failed; were inoperative and some of the water in the drain lines had frozen.Reporter stated there are no clearly defined procedures in the Maintenance Manual for cleaning up water. He suggests that better procedures be developed to drain residual or trapped water from a blocked drain line prior to flight.