Install bigger '83 booster into '80 SC?
 

Folks, since I rebuilt my brakes a decade ago they've been terrible (can't stop quickly / can't even come close to locking them up). Rebuilt the calipers twice, replaced all flexible lines, replaced the MC twice, changed out all pads, and new rotors all around. Bled about a barrel of brake fluid with vacuum, pressure, and pedal with no improvement. Maybe bad luck with a bad MC or bad booster?

So I'm on attempt #3 here to make it right so I can stop in a hurry. I'm replacing the MC again and was going to also replace the booster. The used good condition booster I just got is from a 1983 (Ate 3.6842-1002.4) and I notice its a bit bigger than my 1980 SC booster (3.6832-1001.4) but still fits. Can I run this one with my 1980SC master cylinder and standard calipers? Everything seems to fit fine.

What brand brake pads are you using?

I don't recall the brand but they were bought from our host. I have just bought some mintex and a second pair to experiment. I was successful in getting the replacement booster in. Bleeding is tomorrow.

I have seen this issue before, on a 82sc I drove the car and the brakes were crap, he had done everything and beyond what you had done. We removed the pedal box and found that the bushings were non existent, have you checked yours?
All bushes replaced and the pedal and fell was superb.
Remember it's always the simple stuff that counts.