Last weekend I'm so depressed that my EC-TL got a messed up curtain issue as below shown. The curtain just kept open and mirror's not returned. Eventually I fixed it and would like to share this experience.
The reason to this issue is that the T3 spring is jammed. It's a spring inside the body and hard to reach. You have to raise the curtain and spot with a flashlight to see it. Then use a tiny flat screwdriver to reach T3 and then carefully "shake" to unjam it. There's a curtain spring tension adjustment on the right side of body, just below the leather. Though it may not be necessary to do the T1/T2 tension adjust, still I made it less tight a bit. I assume that T3 is jammed is because of the tension of T1/T2 are just so tight?
Below are pictures of the process. (sorry, I misplaced the T1/T2 on the first picture) Hope it helps if you got similar issues to the Bronica EC!