My son recently inherited a 76 D100 project truck that was in pieces. Someone had installed a master cylinder for a 78 Aspen with braided lines to it. We figured out the master would never work due to the pushrod on the booster being too long, so we got the correct master. It has the smaller reservoir/larger line fitting towards the rear. Now were not sure which line goes to the rear brakes and which goes to the front? It is a disc/drum truck. Any help sorting this out would sure be appreciated.

normally the rear of the cylinder feeds the front brakes

So it has nothing to do with reservoir size?