Having dissembled and resembled the soft-fill valve that was leaking (seems to work, touch wood) I noticed that in addition to the actual inlet valve, there appears to be another valvey-type fitting at the top, with what looks like a fitting for a pipe ?
Removing the fitting (it just unclips - the whole thing is nicely designed) there's a spring, a white washer/valve, and what looks like a tiny elastic band underneath ?
When I let water back into the assembly there was a tiny weep from the cover before it stopped ?
Not being au fait with plumbing design, it's intrigued me.
I could email Fluidmaster, but it's more fun to ask here :)
It's a 747 side-entry model, if it helps. I couldn't find a clear image.