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On closer inspection I found the problem. The diaphragm is fine.
Inside the petcock is a small metal piston with an O-ring around the end of it. The piston is mounted in the center of the rubber diaphragm. The end of this piston is held against a circular opening inside the petcock by a spring. Fuel flows through this opening, from the tank to the engine.
When engine vacuum is applied, the piston is pulled away from the opening and fuel flows. When the vacuum goes away, the spring pushes the piston back into place and the flow stops.
Here's the defect:
At the base of the opening is a ring of white packing material that looks like teflon. This is what the end of the piston seals against.
On mine, this white ring is defective, preventing the piston from seating.
This is not a replaceable or repairable part. You need to replace the entire petcock.
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