No Reverse 95 S420 722.366

Last night, pulled into neighbor's driveway, put her into reverse to back into my garage and nothing. All forward gears work fine, flawlessly, actually.No indication of failing bands and clutches.

I've spent the better part of my morning reviewing threads and pdf's relating this common problem with the reverse seal tearing or o ring in valve body? Service bulletin says check plates and sealing rings on brake B-3 piston...

309,334 original miles on the drive train. I suspect the seal finally wore out.

Please throw some opinions over here.