I think I may have found the problem

Had another look at the diverter valve and on second look I think I may have found the problem. (probably didnt help the first time i took it apart I was hanging out my arse)
Am I right in thinking theres supposed to be an o-ring in that groove?



My gauge at idle sits at -22hg !? although Im sure it has always sat this low prior to this flutter situation!?
can anyone comfirm?

thanks