I am located on Long Island , NY and have a 20*40', 3.5'-8' deep vinyl lined inground pool with 4 wall returns and 2 deep end returns in the floor. The 4 wall returns and 2 deep floor returns are valved independently of each other. There is one 2" pvc pipe supplying the the 2 floor returns a separate 2" line supplies the wall returns. Using a 4.5 hp shop vac I was unable to get any air to blow up out of the floor returns. The shop vac cleared all other lines easily. Since there is no way to prevent water from flowing back into the floor returns even if I can blow them out , how is this handled. This is my first time closing this pool myself. It was previously closed by the pool builder without issue. I am thinking that the water level in the deep end return line will return to the same level as the pool water which is up to the bottom of the skimmers, about 1' down from the coping. I have a loop lock mesh cover. The shop vac displaced some water in the line since none is visible where the pipe comes up to the equipment and I poured a gallon of pool antifreeze down the pipe. I am concerned as to whether this is adequate. Thanks in advance for your help.