Draining your loop in a MM case? ' By coincidence, I just spent 3 hours yesterday evening doing the same in my Horizon case. I thought I'd laid the loop out for easy draining, but it was still a PITA.
The short answer is that draining the loop involves popping a tube off somewhere and letting the liquid flow out. Continue until no more liquid is left.
If your reservoir is at the top, try to identify a tube near the bottom that can be taken off easily. Figure out how to get a container underneath to catch the liquid (and make sure the container is big enough to hold the liquid, and that it's not used for food/drink), which may require some manuoevering of the entire case. Once everything is in position, unscrew the reservoir's fillport so that air can enter the system to replace the liquid that leaves, and pull the tube. Have a bunch of paper towels nearby, just in case. You'll probably have to tilt and rotate the case a few times to let all the fluid out. Bet you're regretting that MM case now, eh?
If you're lucky, see if you can detach all the water-cooling components and remove them from your motherboard as one intact loop. This often isn't possible, but sometimes you get lucky. Then just take the whole thing over to your sink and pull it apart (detach the pump first, so water doesn't get inside) without worrying about getting any on the electronics.
If your having troubles, feel free to post a picture. No two loops are the same, after all.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133
Use a compressor if you have one, but don't force all the pressure through your loop as your components may pop off spilling liquid all over the system. Maybe a bike pump if you don't have compressor. Or you could blow really hard but stop if you get dizzy otherwise you may pass out.