Hi Goldenrivett,
I agree. The problem is, de ice agent can freeze up in the air as well. So I was wondering if pressurizing HYD system before being sprayed could have prevented or reduced the chance or amount of de ice agent trapped in the opening. As far as I know, there's no difference. With or without HYD pressure, the size of opening between the flight controls and wings/tail remains the same in my opinion. But I know I don't know everything about my Boeing. Thus I am here to ask for help.