I tossed the sink hook up years ago and wound up with about 3-4 more of them after as well without the sink stuff etc.My opinion is they never work right when used that way.I just go old fashion,put it in the tank and fill the tube,then start the siphon by mouth. You will have more than enough time not to get it in your mouth and gargle fish poop if you just watch it.You can actually feel it start anyway. The only other problem I encountered is some of the tubes at some point had a flapper added to the inside for some reason?I guess to prevent back flow? Who knows,but I rip that flapper off with a needle nose plier and that takes care of that flapper issue that used to slow the flow and make it clog.Siphon it out the nearest window,door,sink,toilet whatever you can get away with and forget the sink crap.The only thing that is really useful for is filling it back up but in my opinion, a bucket is better anyway so it can be treated with water conditioner each bucket before putting in the tank, or you have to use double doses when its in the water. That's a PIA. Filling buckets and dumping by hand is like my daily workout.My water bill is high enough without constantly running water also and wasting it with that sink adapter thing.
EDIT: I'm surprised all of you use that sink adapter. Don't bother with it.If you do a manual siphon the vacuum is way stronger and it could actually suck gravel completely out of the tank if I let it, using the regular siphon effect.