ugh... don't try.
My suction tank I just use (obviously) for waste oils, so I dont bother cleaning it, just the end of the tube so none dribbles out as it re-flows.
For transfer devices, if I absolutely have to, Ill run a bit of the 'new' fluid that is to go through, to waste, to flush out most of whats left there. Typically though I use the suction device to remove oils, and buy the cheapest of cheap pumps for the oils to put into different things (diff, trans, different cars that take other diff/trans oils, etc.). By buying the absolutely cheapest ones, I can buy more of them for the same money and not worry about anything.
If you really must clean it, dont use water.. Id suggest you take gasoline from the can that you use for your mower, and suck it in and out of the gas can. A short run will solve and clean the pump, and the remaining gas will evaporate in time. I suppose if you have an old enough car with a non-suction proof gas filler, you could just suck a liter or so out and that would be sufficient to clean the pump.
JMH