Maby this weekend I'll put together a video since there seems to some interest in this.
It actually works exactly like an IV. The valve stays closed until the syringe is attached. Then opened and there is actaully very little pressure in the line. (Since it's just hydrostatic pressure from the inlet method.)
It's enough to barely creep the plunger up on the syringe and I actually let the plunger fillnall the way up with water before I inject it back in to get the air out.
Then I press is down to inject and then close the valve.
Here are some better pictures of that set up. I have it set on the auto doser now so now I just leave the valve open all the time and the doser pump acts as a check valve to prevent any back flow. Remembering to dose 2 times daily became a massive pain. That's why I really liked the doser so I dont even have to think about that whole part.