And then reassemble the unit, when I turn the pump back on the skimmer
goes
crazy for about 5-10
minutes just foaming and overflowing into the collection cup, so I have to
open the gate valve all the way
to prevent this.

Next time when this happens, plug up the air intake instake under the water
level drops down. ;p

Just about every time often when the skimmer is restarted, the outlet pipe
work is still contain lots of air hence prevents a proper solid out flow
from the skimmer, hence makes the foaming within the skimmer to be higher
then normal.

jc