If the choke is being pulled the wrong way by the servo, won't reversing the servo solve that issue. Also when soldering barbs to the tank brass tubing, the heat generated by your soldering iron should not be that hot to damage the tank's rubber stopper, unless you are using too much heat. What I do is first install the correct length tubing I plan on using in the stopper one at a time, pushing it as far as possible to one side so as to keep the iron and heat as far from the stopper as possible. Solder on the barb on that end then when cooled push the tubing through the stopper to expose the other side and solder on a barb on the other end. Then on to the next piece of tubing until all finished.

Karol