Winterizing! this is CrAzy! Antifreeze blow out...

Here's the booboo I did when winterizing a few years ago: I put the little filter screen in the city water inlet, back into its spot backwards. The cone of screen should face outside - if it faces inside, it pushes on the release button for the check valve and holds it open. So when you turn on the water pump, water comes out the city water fill. (smack forehead)

Needless to say, if that's the case for you - its a REAL easy fix. Turn the screen around.