You seem to be going through the exact same thing I am right now on my 1980 Chevy K20.
I've:
- Rebuilt rear drums brakes, both sides
- New hard lines on rear axle running to drums
- New master cylinder, properly bled
- New front calipers
- New front pads
- New front hoses
I've tried bleeding multiple times at each step, still crappy pedal after replacing all that. I am also suspecting something with my rear drums. The front are bleeding nice and seem to make good contact when pressing the brakes.
My rear drums are worn down pretty good and also might be out of center. I redid the rear drums many years ago so unfortunately don't remember what P/N shoes I bought. Drum cylinder seems to be fine as well testing without the drum on.
At this point, the only "quick" fix left is replacing the rear brake hose, maybe it's collapsed? Otherwise I think it's time for new drums and shoes again.
But after pricing out the cost for drums, etc., I might just do a Lugnut 4x4 disc conversion. Tired of these drums.
I feel your pain!
Here is a pic of my rear drum rebuild for reference
