Brake lines plugged or both rer wheel cylinders bad?

[65ChevyC20]

I recently picked up another 65 C20 that hasn't run for a few years. I swapped out the master cylinder, single circuit, which I did bench bleed before install. I crawled under the truck to bleed the right rear and couldn't draw any fluid through. I was using a vacuum pump and then switched to pressing the brake pedal to see if I could get fluid back there using the old method. Still no fluid at bleeder.
I switched over to the left side and got the same results using both methods, vacuum pump and depressing brake pedal, and still no fluid.
I then moved up to the front end and was successful at getting the brake fluid through the front brakes. This truck still has drum/drum set up.

I then went back to the rear brakes and opened up the left and right bleeders and still couldn't get any fluid to flow. I then closed off bleeders and opened up the brake line where it splits to go to the left/right drums and I could force fluid out at that point.
I'm at my wits end here. Is it possible that both rear cylinders are bad? Or a possible plug in the lines after the splitter.
After putting away my tools I decided to see if I could spin the rear wheels, both were locked up. Not sure if this is rust on drums that is keeping wheels from turning or not.
Just want some input before tearing into this truck again this weekend

Wheel cylinders are $14.00 bucks each, not a big expense, if that is the problem