Here are the parts inside a MPS

Here is the diaphram installed into the cover

cover with the spacer

inner stop installed

They crack right at where the spacer holds the diaphram to the cover. Eventually, they crack all the way around separating completely.

thanks Geoff!

so this is where they crack?

the diafram the only part that fails?

Yes, thats where they crack.
Sometimes water/rust get into the other end where the electromagnet is and they fail (rarely).