Searching through Rock Auto's site, it appears that both should have a 48-mm bore.
Someone correct me here if I'm making any incorrect assumptions:
Probably not:
* Rear brake line kinked or otherwise restricting flow.
* If this was the case, holding the brake pedal longer should cause the rear pressure to eventually increase to the same as the front, thus the rears would eventually receive full power.
* Rears never receive full power.
* MC rear piston is seizing before the end of it's stroke, thus rear pressure never builds high enough.
* This may not make sense because if the rear piston seizes, the front should also be stopped as there is brake fluid between the two. If the rear stops, so should the front.
* Seal between MC front and rear pistons is bad.
* This probably doesn't make sense because if the seal was bad, pressure should equalize between the two pistons anyway.
Possible:
* PV needs to be changed
* PV decreases pressure to the rear under hard braking conditions to prevent lockup. Perhaps the PV installed is simply too light and causing the rear pressure to decrease too early.
*This could be tested and confirmed or ruled out by removing the spring and rubber cap in the PV, which should disable the PV entirely.
* Seal between the reservoir and the MC rear piston is bad.
* This might cause the pressure in the rear to decrease, while fluid gets forced back into the reservoir? - Can anyone chime in on this one?
*Only way to test would be to pressure test each line, or replace the MC.
Probable:
* Rear pads/rotors not creating enough friction.
* I AM still using the old stock KJ rotors, but did grind it down nice and shiny and installed new pads.
*Only reasonable test is to replace rotors.
* Front WJ calipers require less fluid volume to provide the same clamping force as the rear KJ calipers.
* If this is true, the fronts would always have more breaking force than the rears, at all pedal travel.
* Could confirm by checking rear brake line pressure against front brake line pressure, as they should differ because the MC front/rear pistons are ultimately going to be at the same position.
So seems to me that the best course of action is to pressure test all four brake lines right? They should all be equal except under very hard braking, and under hard braking, the rear pressure should drop off. Knowing what pressure the drop off occurs, should tell me if the PV is too weak, and/or if the MC rear piston is leaking to the reservoir, and/or if the front caliper volume is just lower than the rear. Yea?