Our 310 does exactly the same thing.
Our seal does work - most of the time, but as I've been instructed in the manual and here:
Turn off external water pressure (pump off or pipe valve off)
Open sink/hot water and let out pressure.
Fill toilet with hot water
let water sit for about a minute
Open valve all the way
press down on the black seal pushing it down towards the bottom of the toilet.
Note: Don't ask me how I know it, but sometimes the seal will come away (it's a snap in single piece seal) and with the valve open, you need to snap it back in place, and get it back into it's original position (yes, it's a pain, but with gloves on, you get the feel for it).
Close the valve and pour more hot water in the bowl until the water is room temperature (maybe 10-15 minutes)
Works like a champ every time for a few weeks, and then it'll start to slowly leak again.
I like the toilet, hate the seat the seal is functionally painful...