So I am new to care of my parents pool, and I have notice the skimmer is not bobbing, I replaced the door, the filter runs at about 12 psi, this is constant. It is a four foot (48”) above ground Sand filter. I have back flushed and the psi is unchanged. The door will bob with filter off. When filter on, if I manually ajar the skimmer door Or remove the skimmer door the psi Drops to ZERO and the clear part o. The filter system has air in it and the part the send water into the pool has full force causing bubbles, as soon as skimmer door is replaced it let go it sucks back into the skimmer. Thank you in advance from the newbie!