Master Cylinder question.

Yes but think of the picture shown above. The first hole fills the cavity then as the piston compresses the check valve is covered to compress. The piston needs to be all the way back for the fluid to get pushed back into the reservoir on the return stroke. Just saying. Fluid control. Once on the backside of the piston would lock it forward and not able to return. Just staring a need for a one way check valve for the return flow.

@AJ/FormS is spot on.

How many master cylinder’s have you disassembled and rebuilt that had “check balls” in them, and what was the application?

Because like AJ, I have disassembled and rebuilt master cylinders, and I haven’t seen one with a “check ball” in the location that you say needs one. Residual valves on the line exit ports yes, but not a check ball. Certainly not on anything relevant for these cars.