IM guessing here but,

the front calipers require alot more fluid to actuate than the rear cylinders, so the rears are acting FIRST.

what if you added a residual pressure valve on the the front line to hold some volume of fluid in the calipers? so that a slight addition of fluid would actuate them as well as the rear cylinders?

usually there are systems built into the master cylinders to counter act the different amount of volume required, maybe a restricter in the rear line, or different sized ports ( front/rear)

if you could discern any differnce in output of the 2 circuits on the MC you could reverse the hookups, connect the front brakes to the rear port of the MC and vice versa.

just some ramblings