I decided to learn how to close and winterize my pool for the first time. Everything went well, bought an air blower that hooked to one of the skimmers and easily cleared out the water from the other skimmer, all the returns, and the bottom return. My question is with the main drain. I have a suction limiting vent pipe, and once all my returns were plugged the vent pipe started shooting out water. I thought this was great until after 20 minutes it wasn't stopping. Eventually i gave up and just plugged the vent pipe and saw bubbles coming from the main drain, i went ahead a locked the valve to seal the air to the main drain.
My question is, is this enough to ensure all the water was ejected from the vent pipe?
How do i make sure the vent pipe is cleared?