the little rubber piece thats on the end of the master cylinder pushrod?

Norse said
As an add on to this; thanks for posting about that rubber piece.
I'm replacing the master cyl for the first time on my Barracuda (w/75 Dart manual front disc.) and didn't know about it until after I torched the old one out.

I too just found out the hard way too, I shredded mine trying to remove it.