Not too sure why people are taking the time and effort to remove the spring and seal out of the tip over valve to make it hollow? Just remove the valve, insert a double barb brass fitting that joins both side of the hose back together and done.
Not to say the extra effort dosen't get the same results but I chose to just revome the valve and join the two ends together. I didn't want to risk breaking the plastic or creating a leak when I put it back together then having to take the tank off again to repair something I may have caused.