1972 C20 master cylinder

[KLASS6]

Hi everyone,
I bought a disc/drum power brake booster from POL and the prop valve bleeding tool.
Can someone confirm that on a 71/72 3/4 ton trucks the front reservoir of the master cylinder is for the rear brakes and the back reservoir is for the front brakes?
My mechanic do not works often on US car and he is not sure if has to plug the pipes this way.
He has plugeg front reservoir/front brakes and rear reservoir/rear brakes and he has trouble bleeding rear brake even with the bleeding tool for the proportioning valve...only few drops of brake fluid are coming through the rear brakes.