Master Cylinder "Locking Grommet"?

Dana67Dart said

Inside the piston of the master cyl. Assuming your master does not have one now.

It holds the pushrod in place.

Note the manual brake application note

Click to expand...

I think power brake master cylinders and pushrods don't have the provision for this grommet (not necessary with power brakes). With all the mixing and matching done over the years, your manual brake setup may not be able to take this either.