I finally decided to disregard the service manual and remove the brake safety/bias valve assembly as bleeding the brakes resulted in the pedal becoming solid halfway down its travel and not really doing much. Bought an open ended ring spanner (flare nut wrench) and some PlusGas releasing agent. Sprayed it all up and let it sit before attacking it with the wrench.
After a while it was all undone.
Offending item out. It’s not very big, but it’s milled from a chunk of aluminum.
Looking into it showed that it was kinda grubby. Undid the safety shuttle valve assembly.
Yeah, that’s meant to be very clean inside there. It wasn’t.
The shuttle valve end. It was firmly stuck in position, that’ll be why the brakes didn’t want to bleed properly, then.
Had to hammer it out. It was covered in crusty, solidified old brake fluid. Cleaned it up. Surprisingly the seals are still good.
With the shuttle valve cleaned out, I undid the bias valve assemblies, which were similarly dirty but at least not stuck (too much) and cleaned them out.
Reassembled it all and put it back in the car. I now need to bleed the system. Hopefully now it’ll be good.