Servo controlled irrigation valve

I needed a control for my gravity fed drip-water irrigation system. I first tried with very small holes in the outlet but the amount of water was too much for a constant watering.

I didn't find any cheap ready-to-use electric valve that would fit and I also had some servos lying around. I ordered a 5 pack of medical three way valves from Amazon and designed this holder for the valve and the servo. It took me 3 tries to get the measurements perfect but it works surprisingly well now.
The valve requires quite some force to turn. The servo is able to get the job done but it sounds like it is working at its limits. I used the version with metal gears (Tower Pro MG90s), there is also a more popular version in blue with plastic gears. It has the same dimensions but I can't tell if this would also be powerful enough.

UPDATE: The MG90s was to weak after all. When the valve has rested for some time in the same position, it is hard to get out of the position and the servo fails. I have up