I’ll be real honest with you Bob, I haven’t driven a truck with the dual pump or had one apart to see what sort of circuit it has in it or had a gauge on one either.
My guess is that the vacuum from the pump is pulsating and at low throttle settings you would see that low frequency pulsation without the assistance of the manifold vacuum. By plumbing the two vacuum supplies together internally that would smooth out the vacuum signal at low speeds. At high throttle settings the manifold vacuum would be low but the pulses from the pump would be at a higher frequency and you wouldn’t notice it.
That’s just a guess mind you; maybe someone more knowledgeable about the system will have a better answer.
Denny Graham
Sandwich, IL
