Vacuum secondaries are controlled by venturi vacuum, which is directly related to the speed of the air/fuel mixture going past a small hole, which produces the vacuum. Put manifold vacuum completely out of your mind when thinking vacuum secondary action. You can test this operation, and your secondaries, with your air compressor, and the engine
not running. Just blow air past the hole to create the "vacuum." Here's a short how-to, with pics:
Holley Vacuum-Secondary Operation - Tech Article - Chevy High Performance Magazine