I've been having so much trouble getting a good back for my RB-SD.
I have one 6x8 power back (SD style) that has zero light leaks. Over in a span of about a year, I bought S back and SD back, non-power variety from a used source and all had some type of a light leak. I was a bit surprised even an SD type had a leak as I heard (and saw) it uses mechanical light trap rather than foam. Upon close inspection, I did see a use of some foam in hinge area though.
Is this a common experience?? I'd like to have a few back but so far, I'm stuck with just one.