I'm not getting any memory leaks here on a vanilla install. If you have a specific sequence of actions that result in a leak, I'm happy to investigate further and fix.
Open your sys: volume, whatever it is called, pick "Utilitiies", select "copy" from Icon menu, let it make a copy, select the copy, select "Delete..." from Icon menu and confirm on the requester.
Repeat till you run out of RAM.
And yes, Avail FLUSH won't give you RAM back.
And yes, it ONLY happens with ASyncWB.
RAWBInfo seems "clean", it just gobbles up another 120 kB for each window it opens, but that RAM is released on closing, and gives back RAM on exit.