[Merv and Sharon]

For those still interested in this thread and in relation to an earlier comment by Greg re modifying the existing PBR XF system to increase rear wheel braking, I did find this:

Proportioning valves

Some cars with rear drum brakes will have a proportioning valve; these are often set in the range of 150 to 250 psi. As rear discs are not self energizing like drums they need higher pressure settings, typically 500 to 800 psi. Possibly the rear requirements of your car might need a pressure so high that no off the shelf valve comes in this setting. You might be able to delete the nylon spool from inside the existing prop valve so that it no longer works at all, this means that in an emergency you might lock the rear brakes if you press hard enough, but usually well after the front brakes have locked. The acceptability of this will depend on how the rears perform in emergency situations, which means a careful road test.