lol the brake 'vacuum' is actually unmetered air entering the maf circuit. so it will always make an engine run lean when you depress the brake because brake booster air comes from after the maf. that is completely normal.
You just need to start with a pressure test. Boost leak test the system from JUST after the maf (PRE turbo) all the way through the intake manifold to find all the leaks. Make sure you hit 15psi or whatever you run +
And make sure you do NOT pressurize the crankcase. Pull the dipstick and disconnect the fresh air tube from the valve cover (I plug it with a marker
) If you think air is entering the crankcase seal all exits except the dipstick tube and listen carefully in the dipstick tube for air rushing out. That means air is getting into the crankcase somehow still.