Is there a washer of some kind in the bottom of the accelerator pump well?
Yes, it is used to seal the brass accelerator pump tube at the bottom of the well.
We recommend not removing the fiber washer. It is very hard to repair.
Here is a possible solution if you do take it out.
What I do to ensure sealing is to bead-blast around the bottom of the brass pump cylinder where it joins the iron casting as best I can. I then take some Permatex Threadlocker Green (it is designed to seep in between any tolerance-fit objects); I use just enough that it runs all the way around the outside of the bottom of the pump cylinder. Then I heat it using a Map/Pro gas torch to flash-cure the Permatex, and presto: no more leaks.