Aircraft technician reported hydraulic leak due to improper fitting installed.

Narrative

Inbound green system quantity low; checking history under several ATA Codes found a lot of history on this system over the last 2 weeks. Opened gear doors and found green system reservoir drain valve with an illegal fitting on it. Upon removing the illegal fitting I discovered Teflon tape wadded up inside the fitting that was used to try and stop the leak. The original drain valve was heavily damaged externally from someone using the wrong tools and someone ground down the surface for some reason. Upon removing the leaking Drain Valve I discovered the O'Ring was missing causing the original leak they were trying to cover up. Replaced Green System Drain valve and refilled reservoir per MM and released aircraft.If we have to service hydraulics we should be required to check history; on day 0; Station A had to use 1 gallon to service green and 2 days later it was serviced again and no one checked for leaks. We have to emphasize that using illegal parts and not using MM procedures will not be tolerated. Aircraft Maintenance Manual and Illustrated Parts Catalog must always be followed and complied with. This could have ended in a Diversion and serious consequences to passengers and crew. I believe if you review history you will find a mechanic who drained the reservoir recently and this probably the mechanic responsible for this. Aircraft was flown out of configuration for several flights.