Rick Oleson's solution is to put a small dot of tape on the front-center of the main viewfinder window. See his site for the details. http://rick_oleson.tripod.com/index-165.html
Deteriorating light seals and rf mirrors seem more the rule than the exception with the Japanese rangefinder cameras. What you definitely don't want to do is to try to clean the front-surface mirror, which will only further worsen the problem.