I am testting out viton gaskets for the vacuum chamber.
Viton gaskets are reusable replacements for single-use copper conflat gaskets.
The gaskets are rated down to 10e-8 torr, which is fine for now.
The smaller gaskets installed easily, but the large 8″ gaskets were impossible to install… they fell off the flange.
I called MDC and asked “what’s the trick?”
The trick is a little vacuum grease to retain the gasket!
I ordered APIEZON TYPE M vacuum grease from ebay, also rated down to 10e-8 torr.
I made 8 tiny dabs of vacuum grease:
And it worked!
After a few hours the chamber went as low as 5.5e-5 torr. Totally fine for now, but could be better.
I will continue to use them.
Prometheus Fusion Perfection 
Hi Mark, Just to say that I cut my own rubber gaskets, which I did also for the CF flange on my turbo. I tried the grease ‘trick’ but I didn’t get good confidence that the seal stayed put when mating the two parts together (after all, they’ve just been lubricated!). So I cut little squares of kapton tape instead (because it has strong acrylic adhesive) and used them to stick the gasket in place, dry, onto one of the flanges at several places around the gasket, with just a tiny overlap onto the gasket itself (just 1mm or so onto the gasket is enough, providing you have done this every 4cm or so around the circumference). Doesn’t affect sealing, because the most important bit is to make sure the gasket is nicely home on that knife-edge and the ‘slopey-bit’. I’m not saying this is best for you, just relaying to you what worked for me.
Look out with Viton – something I found the hard way recently: Any exposure to flame (or other plasma) will cause the Viton to produce HF gas, which is quite nasty…
Good tip!
http://en.wikipedia.org/wiki/Hydrofluoric_acid
Deadly chemical.