Starting the overflow box?

If you are using a U shaped pipe for your overflow, simply insert a piece of airline tubing into the pipe and suck the air out.

Ok, I've been doing this for over 20 years. Get a cheap check valve. Drill a hole the appropriate size for your check valve nipple in the uppermost part of your U tube, and silicone the check valve in place. Attach some airline tubing to the remaining nipple of the check valve. Whenever you need to re-start your U-tube, just stick the tube in your mouth and suck out the air, instead of threading the tubing up one end, and yanking it out.