I posted this elsewhere in APUG, but thought I'd try here too. Someone suggested a CLA, and another thought it might be a back problem (the C doesn't take interchangeable backs).
I have an intermittent problem with my Bronica C. I thought it occurred only in cold temperatures, but today it occurred in comfortable temps. Somewhere in mid-roll, as I trigger the shutter, the mirror doesn't return down, and the aperture blades stay closed down. After trying to remove the lens and the helicoid mount, the shutter/or mirror completed its cycle and the aperture re-opened. But then as I advanced the film, I could see that the mirror was flipping up, and the crank would not stop with its typical firm click. Rather, it wouldn't take another shot, but would allow another turn of the film advance crank. Before I knew it, I'd rolled the rest of the roll through the camera. Anyone else have this problem? I've now had two model Cs with this problem.
I can't help you more than again suggest a CLA. Sorry!
I've had a C for years, never had that problem. But did have problems with the ground glass, focusing screen. I sent it to Jimmy Koh in NY, for around $100.00 he CLA's it and made everything just fine. Contact him, send it to him, I'm pretty sure the estimates were free.