Power brake Problem. 1969 plymouth Barracuda

Andy
Just a wild guess here, but I think when you took off the band around the booster, you disturbed the seal of the diaphram in the booster, and now have an internal vacuum leak in the booster itself. You might have to remove the front half and get the diaphram back in place properly. Wosrt case senario is to remove the booster, and rebuild it on the bench, or get a new one. If you remove it just make sure the rod lenght stays the same, ( measure it), so as not to throw it out of adjustment. Clean every thing uo and lube the end of the rod also. Do you have a fac service manual, ?? They will have all that info.
Hope this helps. Dave C.