And it's in the shape of a cup.
It goes all the way around the piston.
And it's got a particular property that when it's pressurized, and
that's what the red arrows are indicating, you can see that it
pushes that cup seal up against the barrel, forming a tighter seal.
So it's got the properties that the higher the pressure,
then the tighter the seal is,
which is just what you want to keep that highly pressurized fluid from leaking by.
The design of these seals is quite complex, and
here I've indicated a seal backing because one of the things you don't want to
have happen is as that seal is pressurized,
that the whole seal just jumps right out of the groove on the piston.
So there's a seal backing or sometimes a collection of seal backings and
then often, you will have two cup seals back to back, so
that it can handle pressures in, in both directions.