I've never had an issue with using a MityVac, but one thing I need to ask:
Did you bleed the front brakes?

As you removed the MC, all 4 wheels need to be bled. You may use a full quart of brake fluid to do it right if you have ABS. Bleeding my Charger's brakes when I took the calipers off for paint was a PITA, I got tiny bubbles for about 10 mins from each wheel.

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Back in the saddle - 1972 280SE 4.5