Harmon brakes in GA can probably handle your booster...

The reaction disc is the little rubber disc on the end of the chrome pushrod. Im not sure the ZX's had it?

Update. It's been several months when I posted questions about a soft feeling brake pedal in my 79 zx. I checked the vacuun check valve and found it to be defective. I didn't even use my vacuum test equipment but instead just removed it and blew and sucked through it and knew it wasn't holding vacuum. I replaced it and now have very good, stock brakes again.