Here is what it looks like cut apart.
The diaphragm is supported by a plastic disc and held in place by a nipple molded into the diaphragm.
The diaphragm itself acts as the valve, making a seal against the case top with a lip.
Here is a diagram for better visualization. If you have a stuck valve you can see where its possible to press the lip region of the diaphragm with a blunt instrument to assist in releasing it (as suggested by noiseguy in the tech docs section).
Interestingly, there appears to be a secondary rubber diaphragm valve just above the vacuum inlet. I surmise that this is present to prevent fuel from draining into the vacuum line in the event that the main diaphragm becomes punctured.
