priming help

Just installed a 500 cu. in. priming pot for a Sequence 5800 pump.
Before turning the pump on, I flooded the line and made sure there was
water in the pump and there was water up to the top of the little dome
on the priming pot lid. When I turn the pump on the water is sucked
down a bit leaving a little air in the top of the priming pot. . As
time goes by, 10 - 15 minutes, there is considerably more air in the
priming pot now and the water level in the pot is down a couple of
inches. Is this normal? Or should the pot remain completely filled
with no air in it except around the very top surrounding the little dome?
There are two (2) 2" Tetra bottom vacuum drains feeding a 3" pipe that
goes to within 3' of the priming pot where a reducer takes the pipe down
to 2" which feeds directly into the priming pot. The outlet is at 2"
until it is reduced down to 1 1/4 " into the filters.
Thanks for your help.
W. Dale