So a bunch of years back I did the full SLH brake upgrade on my Shelby Charger ( Caravan calipers, Daytona rear vented disc, Daytona 4 wheel disc prop valve, and D150 master cylinder. I also replaced all the rubber lines with the stainless ones from FWD.

From day one I've always had a harder than normal peddle feel. It stops well but I have to really push on the peddle to do so, It's almost as tho I had little or no vacuum assist. ( like manual brakes )

Move a few years later and not driving the car until late last summer because of life. I decided to replace the booster and when I pulled the old one I noticed there was a piece broken that could affect it's function, so I thought I fixed the problem... which it did but only by maybe 20% less brake peddle pressure.

Last week I drove a friends Shelby Charger with mostly stock brakes aside from shadow caliper and his brakes work well and his peddle feel is much more what I would expect to have.

Anyone run into the problem and if so what was done to fix it?

Just thinking out loud, could it be the master cylinder causing the issue? These cars have rather small boosters and maybe it could not be proving enough assist to push the large piston in D150 master? I've been thinking about trying a 24mm master out of a daytona...