If I were you, I'd replace the rear rubber brake line running to the rear axle with a braided steel line and see if that helps with the fluid delivery to the rear. The fact that both rear wheel cylinders aren't moving much fluid is a major red flag you have some type of constriction. The lining of a brake line can fail causing a problem not readily apparent because you can't see it.

If the brake line replacement doesn't improve the fluid delivery, I'd replace both wheel cylinders. I'd also follow the metal brake line path from the front to the rear looking for any damage to the lines that could be the reason you have poor fluid delivery to the rear brakes.

But, maybe it's just due to the rust and the bleeders aren't working like they should. Maybe it would be smart to start off with new bleeder nipples. If that doesn't do it, maybe do the wheel cylinder replacements second.