I went ahead and flushed out the brake fluid on our 2005 Impreza OBS today with no issues (using a one man vacuum bleeding tool). Afterwards, I said, "hey the clutch fluid looks nasty so let's do that too".
I opened up the bleeder and couldn't really pull any fluid, even though I had maybe 15 lbs of vacuum on it. I think it was sucking in some air around the threads of the bleeder but no fluid to speak of. I had also suctioned the master cylinder down to the low mark (but not empty) and refilled with a little fresh fluid. It sounded like it might be pulling fluid, but the bleeder tank didn't really fill, nor did the master cylinder level drop.
After doing this for a while, I noticed that I lost the clutch pedal (just goes down to the floor) so air got in there somewhere. I kept going with the bleeder tool but again not pulling anything.
Also, since the clutch stayed down, I pushed it in, opened the bleeder (got a little air and brake fluid shooting out) then closed it and the pulled the clutch pedal up, so I know there's air in there.
So now it's sitting in the garage and can't move... uggghhh.