Rear brake bias

[Stakkabo]

I have built a 172 rally car and have having issues with too much rear brake bias. I have removed the abs unit and have a proportioning valve fitted to the rear line but I still have too much rear brakes. My question is whether a non abs master cylinder will fit a abs 172 and if so will this have an affect? I have heard that removing the abs fuse will give 50/50 braking so I am assuming that a non abs car would have different valving to supply 70/30, 60/40 braking instead of using the abs unit to control the bias. I'm not allowed peddle boxes in my regs or that would be next purchase.