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Light leakage is very common and according to my experience result of a worn darkslide light seal, also called light trap. 50 % of all backs I bought secondhand needed a new seal. You can buy the light seals from your Hasselblad distributor (for less than some people spend on ebay), at least here in Germany.
It is really easy if you don't mess around with a poorly fitting screwdriver. The magazine light trap for Hasselblad consists of two pieces: a foil and a strip of metal with foam. Wen you disassemble your magazine, look how these two pieces are arranged inside before throwing them away. For changing the light trap you have only to remove the forward facing frame plate which is held by a number of small black screws. Do not loose one of them! On older type 12 backs the screws have different length, so do not mix them! Put in the new light trap, fix it by pushing a dark slide against it so that it stays in proper shape while putting the frame plate back in place and fix the plate with the screws. 15 minutes work or less.