It's likely the inner seals. You have an issue that is a copy of mine.
I got a new booster from Classic Alfa and the problem went away. Their p/no BR100R, (now £205)
I sold the old Benditalia booster on Ebay for £75 , as needing a rebuild. That compensated a little for the expense. It has now done 3000 miles + and has been perfect.
1963 Giulia Ti Super Rep