I have had a plumber in 3 times - problem - the flush valve doesn't flush long enough to completely empty the bowel. Even if you hold handle - it still doesn't have the thrust to empty. Bowel is 1938 - the flush valve is about 15 years old - have replaced the blather (leather) - they say it might be the vent on the roof? We do not have any problem anywhere else in house. Other toilet is a tank in the basement. Any suggestions?