Brake question

[Bernie]

I know questions about brake setups have been asked before. I did a search and seemed to get contradictory results. Here's my question. I have a, mostly 1969, K20 with axles off a 1978 so there are discs on the front and huge drums on the rear. I need a master cylinder, booster and prop valve. Someone said that it is better to go with the 1971 3/4 ton setup because the 1978 rod would be the wrong length. I can see this, but on the other hand there were several master cylinders for use in 1978. Which one you used depended on the size of the rear brake drums. So would a 71/72 master cylinder for disc/drum work with all 78 systems ? Or are the rods really the same and I should go with a 78 master cylinder ? Sorry so long , I'm deeply confused. Thanks