reading the original post, I do believe you are correct. you need a smaller bore mc.

here is why.
The amount of force you are applying does not equate to enough force at the calipers to lock up the brakes.
by using a smaller master, the math showed for me an increase of over 100 ft pounds of force applied at the calipers over the larger master that I was using.

since you don't have a booster, this is the only way to increase your brake force applied.

feets excel sheet shows that just by changing the master cylinder, no other differences, makes a huge amount of force difference.

I found that out too real world, that I could lock mine up after that.