So I've got an older hydraulic floor jack that can't keep it up.
I took apart the plunger assembly... hoping to find a bad o-ring.
The O-ring was fine, but at the bottom of the plunger was a
conical frustum (cone with top cut off.) made of some rubbery
material, that has a split in it.
What do I call it?, and where can I find a replacement?
I'm mostly just looking for the right name of the thing.
(rubbery frustum)

The Jack has the following number on it, 649N. Any other ID has
been lost over time. I'd guess it's age to be about 30-40 years.

Thanks,
George H.
(right, I could just go buy another jack...)