I am doing up another carburetor that I have for my replacement engine, the secondary diaphragm cover wouldn't hold a vacuum when sucked on.
The main body had some small marks that were easily removed by rubbing with some emery paper on a flat surface but the cover had a warp in it. When I placed the cover on a machined flat surface it would rock up and down a couple of thousands of a inch.
Without heating it I placed it in the press but I couldn't get the warp out of the cover because it was just fattening it out under pressure but then when I released the pressure it just bounced back. I then placed a piece of common paper under one of the high corners and pressed it again. This was enough to move it past the flat stage and when it bounced back it was flat.
This seamed to do the trick, when I reassembled the cover and sucked on the small hole where the top cover joins the main body it held the vacuum and pulled on the secondaries.
So rather than filling the covers and removing the very small ridge that helps to seal the cover to the gasket it seems that they can be pressed back into shape, they are quite soft and seem to handle it OK, time will tell.
You could probably do this in a vice, however you would want some large dollies to carry the load over the whole of the cover face or there may be a chance of warping the cover.
Hope this helps,
Karl.