I don't understand your reference to pressure. These are all gravity flow devices. There would only be pressure below the water line on the inside of the drum determined by the drawdown of water on the outside of the drum. As it is flowing max flow, with max amount of debris, the drawdown would be max, and the pressure would be the miaximum,, on backwashing, the pressure would go down due to the lessening of the differential depth of the water on the inside vs. outside of the drum.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard