What car has a hold-off valve with rear disks? I don't remember seeing this ever. My family happened to own some non-ABS rear disk cars (GM W-bodies) and there is no valve of any kind present.

Now going to personal experience from swapping from 11.75 disk front and 10x2.5" drum rear to the Dr Diff Mustang Cobra 13" Front Disk and 11.75" Disk rear that I opened my adjustable proportioning valve completely and there is absolutely no brake bias issue of any kind.

Not sure what rear disks you bought but if they are the Olds Toronado calipers I usually see on budget ones, the pistons might be a little bigger than they need to be. An adjustable valve and nothing else (other than maybe a drum distribution block to make the tees) should be fine.