My fix to that problems is I press two retainers inside one another on the press.It makes them double thick.One thing that aggravates that problem is the rear band adjustment being set looser then it has to be,the more the servo moves to apply the band,the more spring tension gets put on the retainer. Believe it or not,higher line pressure has nothing to do with it,the retainer only sees mechanical spring tension on it,not hydraulic pressure. Most good shift kits give you a stiffer spring to put on the servo to get the low band off quicker,that's the #1 reason to push the retainer out!
Note: You can fix it in the car if it didn't hurt the band.
