Thanks, but no, per my post, assist is not the problem. I do not have a big cam. I do not have tall valve covers. I have already tried a hydraboost to eliminate that as a possibility. Trust me, all of the basics have been checked by myself, Musclecar Restorations, and two premiere local classic vehicle shops (one specialized in Corvettes, but its a Corvette MC, so why not). It's a defective part or a complete mismatch of parts, but the vendor assured me the latter is not the case, and you'd think others with the standard Master Power Brakes conversions for these would have the same problem if it were endemic to the package itself.
BTW, an easy way to rule out low vacuum is simply to brake while coasting down. You can make 25 inHg even in a radical car when engine braking, and the vacuum reservoir will hold that high vacuum for a little bit. Easy test.
Air would make the pedal spongy I imagine, not slow. It could, as you note, make the pedal feel slow because you compress the air before you move the fluid, but the pedal is high and hard, rock solid.
I did bench bleed it. Technically I can't be certain the last person to touch it did, but it was the same before when I had, so can't really be it. Plus it'd be spongier. I put the bore sizes and the expected bore size for the front pistons above.
I didn't want to mention it, but this is actually a Canadian-built Pontiac 2+2 427, but it is mechanically -identical- to the Impala and has no Pontiac running gear. So please just think Impala, nothing to do with Pontiac. That's why it's a 427 and not a 428. 100% Impala, same wheelbase, same drivetrain, same chassis, suspension, brakes, everything - same assembly line.
Random tidbit: On that "Trucks" TV show they're converting an old C10 gas pickup to Duramax. They kept talking about trying to save the booster but clearance was an issue. Is there ANY role for a vacuum booster in a Duramax diesel truck??? It made no sense to me.
So, back to my actual question...