[aumoore]

Tom Jones the Mechanic not the singer

Have you tried swapping out the two master cylinders and putting the smaller diameter piston on the front? I am running the Wilwood dual master cylinder setup and they recommended 3/4 for the front and 7/8 for the rear. The smaller diameter piston will give more pressure and less voulme than the larger bore master. If you have a non adjustable balance bar then you have to get less presure to the rear than the front.

This may explain why the rears start locking up before the front as they are getting more pressure due to having the smaller bore master cylinder.

As for volume being a problem I doubt that would be a problem with disk brakes as they actually never stop touching the rotors. Drum brakes are a different story as they back away fully and it takes a little pedal movement before the brake shoes actually touch the drums. If the disk is warped enough to move the pads away from the rotor then they are warped enough to feel when stopping and need to be turned or replaced.

I would just swap out the masters and see if it helps.