1) I was never able to re-pressurize balls that are "far-gone" to a playable condition.
2) However, I made an interesting discovery recently.
For somewhat used balls, when I put them in the pressurizer, I could never raise the pressure above 20 PSI -- the balls would collapse.
If you keep them like this for a day, and then try to play, they will be "broken" balls.
However, if you keep the pressure at 20 PSI for a day or two, you can come later and add more pressure -- and the balls would not collapse.
Several days later -- add even more pressure. In the course of a week ( we had a heat wave ), I was able to raise the pressure to 35 PSI.
When we tried to play with these balls, they had a bounce noticeably higher than a normal, freshly opened can.
In fact, the bounce was so high, that both I and my partner preferred not to use these "bouncy" balls, and use the normal ones instead.
This is just to show that if you raise the pressure gradually , you can indeed increase the bounce of the old balls, and quite a bit!