Brake pedal pressure

[1968CHEVYC10]

I recently replaced all of the brake lines in my truck along with new wheel cylinders and spring kits all the way around, anyway I finally got around to bleeding them. they all bled fine in one sense but the pedal pressure seems really low. the front brakes work ok from what I could tell but the rear brakes I couldn't get to lock up, does anybody know why they aren't locking up? I did re-bleed everything and found no air bubbles so I really have no idea why it's not working. I've checked for leaks everywhere and found nothing. also it's on jack stands so to check them I had someone push the pedal while I tried to turn the drums. Thanks in advance!