Gregg,
Don't know if your booster to master return line is metal or rubber but I once had a VW front brake line swell almost completely closed and would not allow fluid to return to the master cylinder after application of the brake pedal.
Jerry

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Jerry -
If you don't learn something new every day, you aren't paying attention!!
1971 108 280SE 2.8 "Ronnie"
1985 300TDT (76,904 Original miles ) "Sadie" (Sold-went back to Arkansas)
1984 300DT "Green-go" (Sold)
1984 300DT "Silver Bullet" (Sold)
1956 220S Ponton
1985 300D (Sold)
1984 300D