I might be way off the mark but I think you are asking too much of your HH cylinder to compress the fluid through all that extra piping. I don't think your cylinder has the travel to compress the fluid enough.

HH in ye olde datto had a line plumbed how Amtrapid explained with a pipe running from the master cylinder (foot brake) through the firewall along the trans tunnel, bias valve mounted next to the gear stick, into the HH and on to the rear calipers via the tunnel.

Oem piping to the rears was removed. This creates the shortest path and puts the piping inside the car away from rocks, roots & ruts.

My HH used to lock the rears and stall the engine if I didn't push the clutch in quickly enough.

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Bye bye betty blue. Hello F6.