Problem: Low rear brake pressure
How I know: Measured at the Brake Master Cylinder (200psi REAR PRESSURE).
(1500+psi TWO FRONT BRAKES PRESSURE FITTINGS)
Checked my friend's rear brake pressure, same m.c. and the result is 900psi.
Tried Solution:
1. Changed Master Cylinder to new one, re-measure brake pressure, still low for the rears.
2. Adjusted brake booster push-rod per specification, checked brake pedal in-cabin.
Video (to shorten things up)