G'day bigwal,
That's the fuel pump diaphragm, as you probably know, and I'd say your problem is that either that pump cover, or the mating surface on the carby body [less likely] has warped.
The solution is to lap the surface/s flat once more - a wetted [plain water is fine] piece of 400 or 600 grit wet & dry abrasive paper, backed by a really flat surface, is the technique to use. Move the part in a 'figure 8' pattern, until the surface is uniformly shiny.
A precision surface plate is the 'ideal flat surface', but a piece of sheet glass is very nearly as good, and a lot easier to find.
