[kiwidog]

Do you think the booster has ever been apart?
You may not have taken it apart but PO may have.
You may want to check
If it is assembled wrong it can cause the brakes to lock-up as the fluid is not draining back into the reservoir when you release the pedal. The diaphragm has to go in a certain way where the four bolts for the firewall bolts go through the booster cover. The diaphragm has what looks like a grommet on the middle of it, the groove has to go onto the booster housing the correct way. If you install with the grommet like part not fitted correctly it has some effect on the push rod length of the booster to master cylinder setting.
I can't remember exactly how it goes however if you follow the picture in the Toyota shop manual that should show you the correct way.
How do I know this?
Well years ago I assembled one the wrong way and it worked but the brakes tended to remain on after they got little hot from use and the dragging created more heat which locked them up tighter.

Worth a try, seems like you have tried everything else.

Hope this helps.

"experience is what you get when you don't know what you are doing"

Thanks jb