Barry knows what he is talking about. Corvettes are a b---- to bleed.
Did you have the MC tilted a little when bench bleeding? If not, the MC may have air. Some of the early Vettes have 2 bleeder screws - an inner AND an outer so check for that too.
You might also try asking the guys at corvetteforum.com. Many of them have had the same problem and you can do a search function or just ask by starting a new thread.