What happened when you turn your pool filter from drain to filter while pump was on?

you most likely damaged or destroyed the rubber gasket in the selector valve. Get the make/model and go down to your local pool store and buy a replacement. The gasket is glued to the selector valve and only held down by spring pressure when the top part is properly seated. If you lift the selector with pump pressure applied to the system it will usually dislodge all or part of the gasket and the selector will no longer seal properly.