booster addition to manual brakes

As to those 9 inchers; they were designed for 13 inch tires, which are very easy to overpower. If you upsize your wheels and tires, there is a whole world of difference in stopping power available to you, with bigger brakes.
The spindles that go with those 9 inchers may not be the strongest either. Back in the day, they were known to break under heavy loading.It even happened to me. I will never run those on anything I drive. I have no idea how common it was.
I installed 295 tires on the rear of my A. Then I was able to increase my rear braking power through wheel cylinder sizing, elimination of the P-valve and drum size/ shoe material.With the KHs up front it stops pretty nice.And finally the back stays there.