I meant to say the water is spraying out the grooves on the wide plastic rim (not the piston itself). Thanks again.

Turns out it's a 1B1 Master by Coast. I think I got it figured out (pull up red tab, then unscrew top, etc....)

I would be inclined to just replace them.

400As are only around $10 each.

Not worth hassling with off brands or trying to even find the parts.

Agreed, and that's what I'm going to do.

But since Toto's are supposed to be so wonderfully amazing, I wonder if there was supposed to be some advantage to this thing.

Fill valves have relatively little to do with flushing performance.

They only start to operate as the water level falls.

Getting the water from tank to bowl is what determine flushing performance.