Hello Paul,
Beyond what Jon said, I'd bet a majority of the early Camaro Trans-Am race cars ran without a brake booster. Unless set up just right, assisted brakes lack the "feel" of a full manual set up. With an over assisted power setup it can be too easy to lock the wheels under threshold braking.
Of course the Penske/Donohue Camaros made power brakes work, and so did some independents... but I still think the majority of the rest ran a full manual set up.
-Chad