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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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k