According to the guy that does the engineering inspection for the mod plates, you aren't supposed to get rear lock up at all. All 4 wheels are locking up at the moment. The bias valve to the rear is wound all the way out (least pressure to rears). Not sure where to go from here.

screw the bias valve in a bit more

think mine was screwed in about half way when i had them on the car, that was with landcruiser calipers on the front

did an r31 rear brake setup for boofhead and the bias was fine, didnt change the master cylinder or anything. thats running twin piston sumotomos on the front

Posted this question the other day in the build project forum but no joy with a response.

I have a CA18DET conversion in a 1200 ute running R31 rear brakes from a 120Y master cylinder and a Wilwood proportioning valve in the line. I also removed the valve from the master cylinder that's there for the 120Y drums. I'm still getting rear wheel lock up. Has anyone done this before and managed to solve the problem?

Thanks in advance.