Brake booster, adjustment rod, or something else?

[zac]

Went through brake system on my '70 K10 Burb, still having trouble. Truck was converted to front discs by a PO. Brakes never seemed to be very good, eventually got to where I couldn't ignore them anymore. Rears didn't seem to be working at all, in front pads were worn but everything else seemed OK. Some lines replaced, new MC and proportioning valve. Everything is properly bled. Truck stops, but doesn't seem to have enough authority. Pedal is hard, not at all spongy. Hammer the brakes and it just slows down. Back when I was driving 40's and 50's trucks with a single res MC that would tell me to lengthen the actuator rod. I think that's what I have here. Any opinions? The power brake system seems to be fine, so I doubt it's the booster, but you never know. Thanks.