Master Cylinder not bled

[1968CHEVYC10]

I've been trying to bleed my brakes after replacing all the line, there were a few leaks which slowed things down a bit but I think I've got those fixed. But now I still can't seem to get any good pedal pressure or stopping ability. I was talking to my Grandpa the PO and he said he installed the master cylinder a few years ago but didn't bench bleed it before he put it in. Could this be why I'm having trouble getting pressure? It's a '68 with manual 4 wheel drums. Thanks Yall!!