I too have had a tough time using my MityVac to bleed the brakes. Opening up the bleed screws enough to get decent flow going lets air get drawn in around the threads. Open the bleed screws just a little and I got either little/no flow or had to up the vacuum to the point I got air in around the hose. Obviously some people can make it work but personally I second Rick's comment, use a pressure bleeder and save the MityVac for other chores.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold