It's good to hear that brunico's problem may have been resolved. They are great cameras when they work, but for those of us who have bought used ones relatively recently and have no prior experience, threads such as this one may be part of the knowledge base we need to build up.
I've had two problems with SQ-As, the first was a light leak in a back, which wasn't all that difficult to solve by replacing the foam seals. The other was the cable release (which is mechanical on the SQ and SQ-A) stopped working, and over a few more shots, even the release button on the front stopped working. It's probably just a CLA type of problem, as the lens, back and finder were OK on another body. I actually acquired two additional bodies after that incident, in order to have a spare. At $80 or $90 apiece, it seemed the most effective solution. So now I have almost three cameras worth of components to do substitution for trouble shooting. Some rainy or snowy day I have nothing to do (ha!) I may try and open up the unhappy body.
I have some trouble envisioning how that could happen, but I suppose it's not impossible. I have seen some Bronicas where it appeared the film back had been taken on and off about skeenteen million times and some of the hooks and catches were a bit worn, maybe an edge or notch that should catch solidly has been rounded off and needs replacement or a bit of tweaking.. . . the back opening with the pressure of sunlight on the catch