Well, I tried this with a 25 guage needle and a vaccum pump. Just a warning to all do not fucking try that it aint worth a fuck. I kicked th sonofabitch up to 5 in. Hg vaccum trying to get the shit to come through. I got about 3ml and the needle moved and closed off against my vein wall and fucked something up. Vein ballooned up under the skin so I pulled out the needle.

Now... 25 guage is too damn small for blood letting. By the time you get blood to pass through the needle for piss, you will be at an unsafe vaccum level.