Bronica ETRS eats batteries.
From my experience, generally when cameras eat batteries they have an electrical fault, if it was mine I would get it checked by a qualified camera repairer.
I have to agree with BENJIBOY, as I have had similar frustrations with dead batteries.
I found that the fault lie with the lens, not the AE II finder. Whether the finder was installed and not turned on or not, or not installed made no difference.
Removing the lens while the camera was not used/stored kept the battery from being drained.
The lens system defaults (no electrical power) to 1/500 second shutter speed and uses electrical power to keep the shutter 'open' for longer durations (1/250 through 8 seconds).
I decided to replace the lens with another (KEH) and haven't had the problem since.
Another alternative solution might be to select 1/500 second shutter speed and AE prism to M (manual) if not OFF.
Good luck
Otter