Hi,
My friends 83' Camaro is having a braking issue. The Camaro had installed a used but in good condition LS1 master cylinder and LS1 booster. It's using the stock proportioning valve, brake hard lines look good and without bends or pinching. The rear calipers are new and also the flexible lines were replaced with new stainless steel braided by Earls. Front calipers are used but also in good condition LS1 swap.
The problem is the following; brakes are HARD and it will only slow down the car, not even one of the four tires is braking hard, the car just rolls slowly until it stops even when slaming the foot into the brake pedal.
The complete system was gravity bleeded, manually bleeded and even vaccum bleeded, nothing, not a change.
Vaccum line seems ok and also the PVC valve looks ok.
What the heck could it be???!!!!
Thanks in advance!!!!

 

IDK my first thought is bad booster honestly sometime it's the simple things when you least expected
Next is vacuum but you said it was good
It's the booster

 

Never paid much attention, but I think the vacuum hose connects to a check valve at the booster. I'd check function of that before pulling the entire booster.