I have a Toto toilet with (Model #??) "C854" stamped on the inside of the tank). It doesn't fill the bowl after a flush unless I hold the lever down until the waste is discharged. It only fills to half (or less) the proper height if I release the lever immediately. How do I fix this problem? There doesn't seem to be any adjusting mechanism on the either the flush or fill valve that have any affect on it.