I have a 1974 pickup, c10 stepside with factory air. When are the kick pad valve and plenum valve supposed to open? I would guess they are supposed to open when the heater control is in the vent position, (they don't), but I can't figure out the code in the shop manual. Also, would a vacuum gauge be a good way to check whether the control line is getting vacuum to the actuator? Thanks for any help,
ricky