After doing some digging, I found out a couple things about the original AD master cylinders. First, the piston end of the push rod is ball shaped (not just round on the end), and 2.) the end of the master cylinder has a sheet metal plate with a hole in the middle that the push rod passes through. This plate is held in place by a wire spring clip. So, the push rod is captured inside the master cylinder housing between the piston on the end plate. This set up is what makes it possible to adjust the position of the brake lever by shortening/lengthening the push rod at the clevis.
Your description is what I was thinking about...we're on the same page now!
Whatever you make for a bump stop it needs to be robust enough to never fail allowing the rod to suddenly pull out...rubber could squeeze out or collapse over time.
Mike B
