This is a 11 year old thread but here is what I do.
I've done mine a number of times with a Mightvac and haven't had any problems and really need to do it again.
Have you taken the lid off of the brake fluid reservoir or left it loose before you started? I also break the bleeder valve loose before I begin, not so loose that you might get air into the system but loose enough that there is no problem once I draw the vacuum. I have also never drawn a vacuum before I loosened the valve but have pulled it afterwards but with the line attached to the valve.
Also have a jug to poor the fluid into while you are under the truck and check the top reservoir often so that it doesn't go dry. You can also suck out the old fluid from the reservoir first and fill it with new fluid before beginning.