What wheel cylinders?

First, make sure the fronts are properly bled and all air is out.

Then, 2 paths:
1. Go down in size by 1/16" to 1/8" (assuming you have 10" brakes originally). Tat would probably put you at 7/8" or 13/16". Rear brakes lockup
2. Add an adjustable proportioning valve in the rear line