Unbalanced brakes

Hi all
Today I was driving my Demon on a countryroad when after a turn, I saw a traffic slowdown in front of me. In fact, it wasn't a slowdown, but the cars were fully stopped and very close. I realized tardively and I had to make an emergency stop.
Problem, the rear wheels block and the front ones not.
My speed was approximatively 50mph and I stopped before knocking the car in front of me (with screaming and smoking rear tires).
I would like to know what to do to balance correctly my brakes
I have front disk brakes and 10" drums in rear. Disks and pads ok. The booster and the master cylinder are new, but aftermarket. The proportioning valve seems to be original to the car. I think that the bleeding has been correctly made.
An idea?
:thankyou: