I have narrowed it down somewhat, and have found that the issue is most likely the line-lock that is installed on the subframe and is between the distribution block and the pressure regulator valve.
(Yes, I have mounted the line-lock on the rear system so it isolates the rears from working which gives me normal use of the fronts during a burnout.)
With the car warmed and the rear brakes holding on, cracking the flare-nut on the distribution block under the master cylinder, no fluid was expelled.
But when I cracked the flare nut on the inlet side of the pressure regulator valve, fluid was expelled and the rear brakes released immediately. The line-lock is in between.
This also proved that the flex line, wheel cylinders, park brake adjustment and rear brake assemblies are all OK.
It's a Moroso (44050) line lock and their manual says they are sensitive to heat. They aren't that close to the headers on my car, but the hot air flow from the headers would be warming the line-lock somewhat.
To see if that was the case, I made a small heatshield over the line-lock and it didn't eliminate my problem, but it did reduce it, so I think I'm on the right track. (BTW. The line-lock is creating the fluid restriction all while switched off.)
I will now consider options regarding the replacing, rebuiding or relocating the line-lock.
Thanks a heap to those who have helped me in this thread.