I just acquired a Mamiya RB Pro-S system and film tested it. Unfortunately, there are light leaks. I found a source of one leak because it appears on every frame at fixed distance from each frame. There was a seal a tad too short - obviously an attempt to fix by previous owners. I think this one would be a simple fix. More difficult to track down are two other issues. As part of my testing, I inserted a dark-slide and unmounted the back. Then I let the opening (still closed) face direction of sun for few seconds. When I did this, that particular frame had a faint fuzzy leak around the perimeter of the frame. I think it means a leak from frame opening and the dark slide itself. Also, when each time I rotated the back, there were additional leak - kind of faint but it's there. It appears as a bar exactly half way between the frames all the way across the width of the film. This is less troublesome because it's entirely outside the frame. Here are couple of questions. I ask these because RB are an popular model here on APUG and I'm pretty sure there is quite a knowledge accumulated already. 1) Are these pretty much known issues with known fixes? 2) I can replace seals but other than wasting a roll of film each and every time to test, is there a trick? 3) I'm thinking rotating back is not the cause of this problem? The seal appears intact. Plus, when I rotate, I grab the back. I can see how it can distort the back itself just enough so that weak seal might leak when I do this. I have number of options. I bought this kit from a reputable place and I'm sure I can exchange it. But, because of the age of the equipment, I hate to keep getting new ones and keep testing, wasting a roll each time. I would very much like to have my options lined up before I contact the seller.