Disconnect Master from Push Rod

I replaced the master cylinder on my 72' Duster yesterday and it would have been a relatively simple job if I didn't have to spend so much time trying to separate the brake pedal push rod from the master cylinder. I ended up removing the internal components from the old master cylinder and cutting the guts off with a hack saw. While I finally got the push rod out and got everything back together and working, I know there must have been a better (and much faster) way to separate these two parts.

Does anyone know the CORRECT way to disconnect the brake pedal push rod from the master cylidner?

There is just a little plastic (or rubber) ring that is holding it in there. I tried heating it up with a heat gun and even that didn't seem to work. Any tips would be appreciated for next time.

Thanks,