[wslade2]

I have an E model and this from my service manual for "1968 and on" (open attached, I couldn't get it to show in the body of this message).

It shows 3 lines from the vacuum gauge, 1 each to the supply side of the manifold for each of the 2 vacuum pumps and one connected to the horizon.

I suspect those supply side lines are for the "buttons"/indicators of the gauge to tell which pump is operating and the 3rd line off the gauge is for the the actual suction measurement of the system. Be sure the "3rd line" is hooked up to the horizon and does not have a leak. The Horizon has 3 ports: central supply off of the manifold, ambient (filter) and the suction gauge. The central suction off of the manifold is what spins up the instruments. The 337 suction gauge is a dead end and should be hooked up like that to the horizon (unlike some systems, like a 182, where the suction gauge may be a parallel pass through).