I’m running the stock 4 piston disks in my 66 with rear drums and a manual brake system. I have no issues at all stopping the car even from triple digits. Remember brake boosters only decrease the amount of effort needed to press the pedal but they do not increase the total amount of brake pressure sent to the calipers. Most of that is dictated by the mc bore size and the pedal ratios. Do it right and you can have manual brakes and a pedal that is not much harder to press when compared to power brakes.

Problem is that most reproduction and aftermarket brake boosters are not set up right which leads many to have subpar brakes that are easy to press but do not properly stop the car. Atleast on mustangs. I have successfully converted a few classic mustangs with subpar braking performance to manual brakes and the cars now stop on a dime every time.

Sounds like something was wrong with the system. I daily drive my 66 mustang with manual brakes and it has no problem in the worst of California traffic. It’s just the stock disks with EBC pads and drums in the rear and has no problem slowing down from 100+ mph. All with a pedal that is not much harder to press than a boosted brake pedal. Plus It’s way lighter since I also track and autocross my cat