How is the water pressure in the faucet you're using when it's just turned on normally? If you've got lower water pressure, you get less suction. The one sink in my house that I can hook the python to has such low pressure that I don't get any suction at all.
I've got a homemade python (It's mostly the same, just put together custom from individual parts) that is 100 feet long (my tank is nowhere near any faucets) and I discovered that I can drain the tank by using gravity, just like you'd do with one of those little bitty gravel vacuums that come with ten gallon tanks. I get the hose full of water, pull it out the door to the yard, put water in the tank end and hold it up, and when it is nearly drained, plunge it down into the water and it should start. It's slow, but it drains the tank out with no wasted water, and I can water my trees or my vegetable garden, or if it's pretty clean, I put it in my livestock water tanks. When I refill, I attach it to the sink the regular way, and that wastes water with mine since the sink attachment has a bad leak and I can't find a replacement for it. I don't have to worry about dechlorinating since I have a well, so I just put the tap water right in the tank.