Hi,
Since you have already tried an electric pump, it might be a good idea to check for an internal diaphragm leak in the brake booster. Mightyvac makes a vacuum pump kit with a large fitting for testing the brake booster. The brake booster should hold 20 in. of vacuum for at least a half hour. Also sometimes in a large cam application a secondary vacuum canister is needed. Both Jegs and Summit carry these. Hope this helps.
Chevy Technician
17 years experience in auto/heavy truck repair