....... The reason I like the MityVac is that the fairly small reservoir to hold extracted fluid is a bit smaller than the volume of the master cylinder reservoir, so as the Mityvac bottle starts to get full it is a warning to stop sucking, close off the bleed nipple and top up the master cylinder.
Very good points.
I use a Mityvac too. The only gripe was that the small bottle tends to fall over, so I made a stand for it using a steel plate base and a piece of tube welded on which the bottle sits in, a slot down the side of the tube lets you see how much fluid is in there.
I tend to work on the little-and-often principle of brake fluid maintenance. I don't worry too much about purging the whole system necessarily, just empty the reservoir using the Mityvac, refill immediately, then do a bit of bleeding at each corner (top up as required). If you do it fairly regularly (year or two) the chances that any of the fluid stays in the system very long is minimal.
I once helped a neighbour do his ancient VW Polo, and he couldn't get it to bleed. I fitted the Mityvac and couldn't get anything more than a few drops out. Ended up having to push on the brake pedal to shift the stuff, which came out looking like cafe-latte. I left him to finish the job, didn't want anything more to do with it thanks, goodness knows what state the seals and pistons were in.