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.

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

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.

what front brakes are you running, if all 4 brakes are locking up maybe you should just get better tyres

I reckon you are running 4 spots on the front (or VT/VX commo calipers), and you are using the 50% max bias valve. There are two sorts I believe.

Measure the cross sectional areas of the front pistons in your current calipers, and compare them to a R31 skyline..

I run VR caliperso n the front and commodore VR calipers on the rear. I use a 180B master, and bias using a wilwood valve on the rear line.

Maybe you are still running the 'splitter' valve?

This is what I'm thinking.

PS. I'm not real sure that the rear residual pressure valve in the master cylinder needs to be removed actually.

Stanza fronts.

the one i have is 53% max lockoff i think, and it seemed to do the job with the cruiser calipers. still had plenty left in it to lock the rears off more, had to drive home with rears locked off as far as they could once as i got a leak from one of the rear bleed nipples that didnt go away with tightening the nipple up. still managed to stop ok so it must have been locking a fair bit of pressure off

residual pressure valve doesnt need to be removed, havnt done it in my car or boofs and we both have 4 wheel discs and the rears dont drag

edit: stanza front, shouldnt be locking the rears with them

By 'splitter valve' I assume you're referring to the splitter for the fronts which takes the single line in and then feeds out to both front disks?. If so, then yes I am but I'm not sure how that could be a problem or how you bypass it.

The splitter that the size of your fist on some cars. Is what I'm thinking. If you are still running the stocker plain splitter, then yeah..nothing to do with that.

i got r31 rear and stanza fronts, not running a bias valve nor have i changed the master cylinder, not running a booster either.
i have no drama's at all aside from only getting 1 or 2 good stops outa it before the fronts start to fade.

but its only a streeter, i will upgrade when i get a chance