Earlier Bronica S2 backs have a curtain rolling over the frame of the back. From the service manual, circled in red: only the curtain and roller: the backs I have are, both on my S2 and on a recently acquired S2a, the newer type, without curtain. ------ the problem: I shot a first roll with a back, Negative shows these light leaks: positive of the lower frame: back home put two strong lamps on the sides and shot a roll: the negative: a positive of a frame: the cause ??? : I have never experienced this with a Bronica S2 and have not much idea. The design of the back seems quite light proof to me. The back does mates tight with the body, no rattling when I try some force, but then light can sneak in very thin slits. The frame of the back has two rollers inside, here seen with film holder removed: film holder in place , no dark slide, potential risk at the gap between the film plane and the edges : the service manual says nothing about light leaks with the newer model. the fix: tried a disassembly on a dirty desk I use for whatever not clinical and took shots on the go with my mobile phone. Maybe these could be of some interest, I haven't found this documented on the net- easy to remove the cover of the side with counter, 120/220 knob setting. Lift upper leatherette corners, be careful it can stick quite, a screw in each corner, remove the lower thin stripe of leatherette, two screws there. Screws are brass. The one top left is recessed and had glue residue I had to scrap then I needed a strong loupe in order to find the slit. At first I tried to unscrew while the screwdriver was wrongly placed and as a result brass was worn out but luckily not enough to "erase" the slit. So careful there: under the cover it's very clean. Seems no lubricant was used, or so excellent that it dissolved without leaving residue: next, unscrew the plate. First remove the dark slide by pressing the round lock at the top right corner. All screws are black painted brass and come easily but the two up and down on the slit side of the back. I hold the back with the plate vertically, slit down and did carefully pull from the other end: then put the back down again and lift, taking care these two remaining screws don't jump away. On the inner side of the plate some kind of felt and at couple corners and a side residues of black sealant? (not glue, it offered no resistance there) an insight of the mechanism: the felt on the inner side of the plate did stick like hell, and after ripping bits I just dipped the whole into acetone. Luckily japaneses must have used an excellent paint, it didn't come off. The sealing is certainly not foam otherwise it had been eaten by the acetone, but it's a fabric with a silky side, I kept the three pieces after drying (the two others were ripped when attempting a removal with a screwdriver). I use only the pierced broader one to make a replacement, this is the original one: since no foam was used I will not use either. I opted for a sheet of self-adhesive flocking material: on the utter side of the plate, the one mating with the body, the acetone was affecting the rubber frame, I removed it, rinsed let to dry and it kept its shape and property. Did reglue it with neoprene: on the slit side of the back, there's a long piece of same felt material, the one under the two screws that were harder to remove (they did unscrew easy but didn't come out). Inserting a screw driver in the opening back the dark slide lock, I take it out. The felt is strongly glued around the edge, which has a lip, and it does cover a rod inside. The road is maintained by a simple spring in the back. i had to rip away the felt covering it: that's the simple locking device for the dark slide as well as the guide for it: when positioning it, we see how it works, when looking at the utter side (to the body) in relation with the strong pin protruding from the body. Simple and effective. It beats swedish engineers of Göteborg (ie. Hasselblad). it also means that one must ensure this part is put back correctly, and the seal must stick strong. The flocking felt I use has a strong glue, so god. I put the felt seals on the inner plate, the felt on that guiding rim with lock, pierce holes with a nail through the felt of the rim, put a bit of glue in these holes, screw the rim to the plate and check the insertion and sliding of the dark slide: not sure about the remains of what did look like some black sealant when I remove the plate (4th picture, middle, in that "Fix" section), and wanting to take no risk with glue i opted for liquid rubber/tape, easy to remove if needed, damages nothing: then, of the mechanism: lubricate or not to lubricate? the disassembly (picture #2) showed no residues and when gears are machined and mated at very high precision, not much is needed. These gears are running like an automotive gearbox either, so unsure, but I consider some little bit of some lubricant does easy the surfacing and friction effect anyway... First I generously sprayed, holding the back over a sink in a way residues and liquid run only through the mechanism only, with electronic cleanser. Electronic cleanser isn't damaging, dries fast and the application straw of the spray works like a slow pressure cleaner- Let it dry then I lubricated with three tiny drops of weapons oil (the picture on the right shows the can but I didn't spray, instead i used a needle near gears axles and teeth : then put screw back the plate, the cover over the mechanism and neoprene glue to put the leatherette back in place. Some masking tape, a toothstick, remove the masking tape at once before it sticks to the glue by the edges, wait couple minutes for the excess glue to start drying and rub the excess with an ethanol impregnated cloth. The end result bears not much of the surgery: Results: again put the camera on a tripod, two lamps casting strong light on the side and shot a roll. freshly dried roll, looks good: positive of a frame: Happy.