I changed out the accumulators on my wagon today. The rearend bounced like crazy going over small bumps. It would almost make a person seasick in the back seat.
I thought I would cut one open to see what was inside for my own interest and all the forum folks as well. There is a ring inside that holds a rubber diaphragm. On one side of the diaphragm is suspension fluid and the other is pressurized gas(not sure what kind of gas). When the diaphragm ruptures it fills the whole accumulator with fluid causing the rearend to bounce.

Rubber diaphragm down:

Rubber diaphragm up:

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