Toilet leaking - is it the washer?
Personally I would go to Lowes or Home Depot and get the more conventional older style toilet siphon kit.  You know with the flapper, fill valve, etc.
I am not familiar with the siphon you now have.  Looks like some kind of internal failure of this piece of equipment.  I am not a plumber, but have repaired a many toilets in my time.  Just a suggestion.  Good luck.