Having chased many a brake problem to resolution on many different vehicles I always start with an overall underside inspection, bushings, lines, hoses, frame etc. If all appears well,
# 1 Pressure check at master
#2 Pressure Check at both rear cylinders
#3 Pressure check at both fronts.|
In your case with a single reservoir master all numbers should be the same. If not find out why
Next step is linings, drums. cylinder diameter's etc.
You mentioned Power brakes, some MFR's change MC as well as wheel cylinder piston sizes to compensate for power vs non power.
keep us posted
