Check the brake booster....
The brake booster is brand new from Autozone, I highly doubt it is bad. Plus, if I pump the brake pedal rapidly at idle, there is no fluctuation in idle speed... that is usually the tell tale sign that the booster is bad.
Just making sure! You did cut a hole in the top of the arc of the hose. You have to have a vent to atmosphere.
I appreciate the follow-up!!! I actually cut a "slit" at the top of the hose about two inches long ... is this correct? I am pretty firm on not putting that hose back on there though. A bunch of gas sprayed everywhere including on the headers and I don't care to repeat that lol.
Anyways, an update on my part. I pulled the carb apart last night to see if I could see any signs of something wrong. Pulled the needles out... they were fine (not stuck) and still looked brand new from when I rebuilt the carb a few months ago.
I adjusted the accelerator pump to linkage clearance to ~.010". I hadn't checked that in a while. I also adjusted the idle air mixture screws back to 1.5 turns instead of the 2 turns where they were at before. A lot of the carb manuals say to start at 1.5 turns and turn *inwards* until the idle speed/ vacuum reading begins to drop.
Lastly, I flipped the carb upside down and verified the secondary transfer slot for the primaries. I measured ~.040" which is in spec.
.......The interesting thing that I found was that I think there was too much preload on the secondary throttle plate? I turned the adjustment screw all the way out, then turned it in until it bottomed out and started opening the throttle plate, then turned it 1/8 turn more just to ensure that the throttle plate doesn't stick. I read about this in the Holley Tuning Handbook.
Overall results... a definite improvement. The idle speed still drops, but not nearly as much. I stood on the brakes from 130 and came to a stop. The idle dropped a bit, but there's a definite improvement. I'm thinking the problem is still in the carburetor, either in the idle mixture settings, or float settings, or possibly by adding this part:
"Control fuel slosh during hard acceleration and braking.
The vent baffle is used with the center-hung style fuel bowl. It extends the fuel bowl vent to near the wall of the fuel bowl and can effectively control fuel splash on the primary side when accelerating hard, or on the secondary side when braking hard."