Brake Adjustable Push rod O-ring?

Any help? 66 dart swapped to disc in front, new master cylinder. And the original pushrod seems to long, so I bought one of these Adjustable type from Right Stuff RC14. Looked right, and the rep. said it fits Mopar.
I got it and yes it fits, but it doesnt lock in like the old one did. The old one has a rubber oring that you force in and it lock the rod into the master cylinder. This new one has a plastic thin, clear washer and it does not lock the pushrod in place. Any ideas???
Right Stuff RC14: MOPAR ADJUSTABLE M/C PUSH | JEGS