Hi folks,

Rancilio S26 here. Perhaps 8 months ago my vacuum breaker started leaking in a big way.

I got impatient before the replacement showed up in the mail and went to NAPA, got a correct-sized Viton (I think but it was black not green so maybe Buna-N) O ring from the A/C rebuild section. Reassembled and all was well for a while.

Lately I've been having a slow leak again. I took the vacuum breaker out and noted that the seal has "flowed" and is no longer donut-shaped. Upon further investigation, Viton is not suitable for steam service (but does 1.5 bar really count). Buna is not suitable for temperatures in excess of 250F, which the steam tables in my copy of Marks suggest is really pushing it. EDPM has a similar temperature range to Buna.

Anyone know offhand what these gaskets are usually made of? Silicone? Something else?

I use just a regular stock o ring ,At least 5 yrs old and no leak. You might want to try a copper crush washer. JK

Yeah, I've got plenty of copper crush rings and even replace them when I'm supposed to.
The O ring I was talking about was the valve seat on the poppet of the vacuum breaker.

No leaks with the new vacuum breaker assembly installed. I'm inclined to just cut my losses and chuck the old one, but the curiosity as to what the gasket is made of won't die.

Ya, The small one I have not had good luck with either,I have had good luck replacing with the heavy duty valve though, Providing you have enough room. JK

Yep, you can get viton that has a black colour.

You need an o-ring that can handle the temps involved, if you want it to last for any length of time.

I've used both viton and silicone, with the silicone o-ring lasting a lot longer to date. Of course, the silicone one is more expensive initially.