the only time my toilet flushes all the way is if when i flush it i pour approx. gallon of water in the bowl with the flushing water.the level in the tank is as high as it can be.
I wonder if you have one of the early dual flush cisterns..... try holding the flush handle down as you flush. They worked by breaking the syphon action as air is let into the top of the syphon unit on a short flush by releasing the handle. Holding the handle down prevents air entering the top of the syphon unit,thus giving a full flush.
Pete
July 2007
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