The brake booster vac line does not go the carb but to the intake. There are other vac lines that go to the ac switch, if you have ac. I use WD40 to look for leaks around the carb base, intake to heads and other vac locations. Be careful using a highly flammable product for that purpose. Being one bank is all low on compression, I think you may have a head or valve issue not a head gasket issue.
