yes 1980 4 door manual brakes 4 speed.

alright heres what ive done:

Replaced over the years:

all 3 rubber hoses
rear brake line that goes across the axle to rubber hose.
3 sets of wheel cyl
drums were turned
new spring kit
new rear shoes
lubed parking brake cables
adjusters freely turn and Ive manually adjusted the brakes numerous times.

front I had rebuilt 1980 style calipers and ensured they moved freely, recently went to 83+ calipers but pedal is same issue

replaced master cyl 4 times and currently have NOS acdelco master installed.

only thing original is the prop valve.

it does stop straight but not enough to lock up the tires i dont think. (never tried)

i asked before swapping to 83 calipers if it would affect the master at all and others have said the only thing it affects is the pedal has to travel a bit more. my main problem is sometimes the pedal is high and firm and feel like i could lock up the brakes but the next stop it might be alot farther down to stop and its random. no leaks.