GM used that style combination valve on many body lines throughout the 70's and 80"s. IIRC, if you blew a rear line, you had the potential to seal off the fluid to the rear brakes when the pressure difference pushed the shuttle valve rearward. It could and often did get stuck there. After fixing the rear leak, sometimes the results were no fluid coming from the rear during bleeding. The fix was to open a front bleeder and put your foot to the floor. If that didn't work, a new combination valve was needed.
I replaced MC, Booster and wheel cylinders on all four corners. Properly Bench bled the MC (which took quite some time), gravity bled the system to start with and then pump bled it. My Car has a drum/drum setup, so no valve switch to keep in place. As I was working alone I used a piece of timber to hold down the pedal, but still get some late braking with a bit of spongyness. I might redo the pumping with a second person. If it doesn't get better I will look into the other options, so thanks to all for the input here