do you mean for a head soak? the capping station is usually plugged by nothing more than trapped air below when you pour some solution into it. if the pump stops in a open position it will allow the ink do simply drain into the waste bottle. from what i know, pumps stop in the closed position. there's probably someone here that can refute or verify that. not sure. or if there is an air leak at the pump lines or the pump is old and doesn't completely seal at the closed position then it will let the weight of the ink press the air out and your head soak will sink into the cap tubes. that's about all i know about. it's all a bit new to me too.
i've got ball valves and inlets hanging out the back of my printer. when i want a head soak to really stay. i have my gf go behind the printer and turn the valve when all the air is bled out of the line, thus leaving nothing but solution. i'm about to install some rc three way fuel valves. they will work much better for the whole retrofit. attached is the current set up. this mod took about 15 minutes to do and i constantly use it to purge, draw ink, monitor pump function, seal off lines, clean lines, diagnose vacuum issues. it is a must have.