Like was said - they're called vacuum secondary because the pressure differential between one of the venture, and atmospheric in the dashpot, is what opens the secondaries. They are the better choice for most automatic equipped street cars. But most people don't understand them or try to actually set them up properly. The springs are color-coded. You buy the assortment, and follow the instructions to get one that works as you need it. Usually that's the purple or the light yellow. When they are opening properly, you will not feel them open. If you feel them, they are opening too fast.
