on the Rochester's, the vacuum advance operates from a ported source so generally there is no vacuum from the port at idle. That is why you don't necessarily have to disconnect the vacuum advance on a 235 to set the timing. This is a slightly different set up then for most other motors. Once the throttle blade begins to open, the port is exposed and vacuum is now created at the port and the advance begins to operate. The question is whether the motor is making enough vacuum to operate the advance which is something you have to test because the advance mechanism is set up to advance the timing at certain inches of vacuum depending on the movement and relative position of the diaphragm.
I never had any issues getting sufficient vacuum to operate the advance when running dual Rochester Bs or the Carter YFs I ran for quite a few years. I did have trouble getting enough vacuum from the dual Holley/Webers I use to run and the dual Weber 38s I currently run so I switched to full manifold vacuum (which presents its own set of issues but that is what hot rodding a motor is all about).
So I would run a single vacuum line and then check the operation. A typical 235 advance for say a 55 distributor, the diaphragm begins to move at between 4-6 inches of vacuum. The total advance is 6.5 to 8.5 distributor degrees running from 7 through 10 inches of vacuum. So that is what you will want to check to see if you have sufficient vacuum to support full vacuum advance.
