I have an SWC and have never had a problem with or without a Polaroid back.
likely the seals. any sample shots? The Pola back on my hassy had leaking in the dark slide slot - as did one of my backs...
Save yourself the cost of the seal changing 'kits' on eBay and the like and just call Hasselblad yourself- A couple years ago when I changed all of mine, I got a set of ten seals with the foil wrappers for what two kits on eBay cost. The 'kit' consists of only a tiny foil piece and a foam strip and a printout of the instructions that are already readily available online.-Jason