...and keeps falling out. The machine was just bought used. It was bought new in 2011 and has the numbers 38.0303.10 on the ID sticker. The gasket has crisp edges and curves and is normally flexible: it appears to be new. It measures 66mm OD, 54mm ID, 5mm deep and appears to be black buna or PTFE. The gasket seals well while infusing; it just will not stay in place, that's all. Any suggestions?

Cammie, Thanks for letting me know I am not the only one. I will try going down the route you suggested.

Buckley

That's strange.. it should be snug, never loose, and certainly never fall out. Likely you have an incorrectly sized gasket. Try flipping it around. that might help if the outer side has expanded, it could provide a bit more grip turned around.

Or the gasket is installed the wrong way. They do have a top and bottom. If you look at the gasket it should taper slightly. The fatter part is the bottom, thinner is the top. If it has been sitting for a while the gasket could have hardened and shrunk a little. Gaskets need replaced every now and then. I do them about every year.