in my opinion the easiest way of draining a system is this:
take a "T-Connector"

and put it to the lowest point in your circuit (if you have an Aquatube -or sth similar- mountet in the front of your computer, put the "T-connector" near the back of the computer).
then a little bit of hose and then you take a cap

and put it on the hose.
to drain your circut take off the cap and put the hose into a bottle or something. Then open up the circut at the highest point (e.g. your reservoir) and watch the water run out of your Sys.
if you really want to go for it, you can buy this one:
all these parts can be found in the Shop at "Zubehör" => "Zubehör zum plug&cool-System"