I have a Bak Pak with the washers... I took a look at it, and it doesn't seem to do much. It could be run w/o them as far as I noticed. To be sure, I would call either CPR directly, or DR F&S to check parts lists and see if you should have one. Either of them should supply replacement parts if you do not have them.
Regardless... I would at least phone them and ask. HTH