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

I just went through swapping out the old pushrod for an adjustable one. I will admit that it was difficult to pull the old rod out of the master cylinder because of that small rubber ring, but added extra insurance anyway by placing a bolt with a nylon lock nut just below the rod. The clearance is about 1/8". So all together we have the ring, the dust boot (?), and the bolt. No more fear of the rod possibly falling out IMG_20210506_161739912.jpg