Master cylinder trouble

[kwlskid]

Hello All,

I’m doing some maintenance on my 1972 GMC stepside ‘heavy 1/2 ton’, and am having trouble finding the correct master cylinder. The original was leaking and the 2 replacements that have been tried don’t work right way too much braking on the front and almost none on the rear.
The truck was my grandfathers and as far as I know and can tell it came with front disc brakes, a 12-bolt rear end, and leaf springs. I don’t know if any of that info. changes things but with the confusion on the correct part I’m guessing it may.

Can anyone tell me what part number(s) would be correct to replace the master cylinder, and booster (might as well it’s almost 50 yrs old). And can anyone tell me anything about the differences between the reg 1/2 tons and the ‘heavy 1/2 tons’

Thanks!