Vitons just a material the O rings made out of and still available in various shores. They last longer and more hard wearing and abround 3 times dearer than nitrile.

What size and cross section you after?

Fizzy…..I have spent quite a lot of time trying to make safety valves using O rings and although I had some successes I also wasted a lot of time. Unfortunately I did not make drawings of the successful valves and they are now on models in Australia but if you pm me I am sure I could remember the essential details.

Meanwhile, if you Google …….Home builder Parts – Roundhouse …….on about the second para down is mentioned their safety valve…click on technical info. ….. which uses an O ring with a nice drawing of a section through the valve. Let us know how you get on.

It did for me to within a psi or two

Jon…………… there are problems if the O ring is just left "floating" as it were. The ones that I made had a housing for the O ring, roughly midway in the body of the valve. The O ring, was inserted from below followed by a hollow screwed insert, thus holding the O ring captive. There are some careful calculations to be made as to the sizes of orifices and the stainless steel ball and of course the spring. I did not use Viton and found that silicon rings were perfectly OK and cheaper.

More recently, I have used Gordon Smith safety valve designs and found them excellent but I still think that O rings have potential as they do produce a foolproof seal.