MATERIALS:
It's easy to tell a 1920's to 1950's diaphragm from our new ones- back 
then the only material which could stand up to gas and still flex 
without stretching was 5 or so layers of tar-coated canvas. Today's gas 
takes that tar right off.
Trickier are the Korean War (oops, I meant "police action"!) surplus 
parts that are sometimes sold in flea-markets. They are black synthetic 
rubber, but an earlier formula which does not hold up well today. They 
may echo the tar-canvas styles and be multi-layered. They'll last longer 
than tar-canvas, but still are short-lived in today's gas and alcohol mixes.
Buna-N Nitrile rubber is the industry standard for a fuel diaphragm. We 
go one better than the auto industry today by using nylon reinforced 
cloth, rather than cotton, and doing both fuel and vacuum diaphragms in it.
"Neo-prene" was the name for the first synthetic rubbers made before 
WWII. Since then synthetic rubber has grown to encompass more than 400 
varieties all manner of properties and uses. Now the term "NeoPrene" has 
become the stuff that wet suits are made of! "Viton" is another 
synthetic for use in fuels, but it does not bond to fabric. Diaphragms 
must be reinforced with fabric, or they stretch out quickly and fail to 
pump. Viton is used for the plates in today's check valves, but it 
cannot be used for either a diaphragm or a pulsator gasket.
Rubber is white to pink-ish, and is colored to make it look more 
attractive. Foreign diaphragms are often red.
In the US we like our rubber black, so the colorant is carbon- or "lamp 
black"(essentially soot) mixed in among the long rubber fibers. That's 
why a rubber boot leaves a heel mark on the floor, and why the top 
surface of your old diaphragm may look grey, not black as gasoline has 
washed out the lamp black over time.
Cork gaskets were used in the old days, and still seal better and longer 
than solid rubber ones. Solid cork gaskets dry out and become fragile. 
Today's cork is a rubber and cork mix, giving both rubber's strength and 
cork's compressibility to make a seal better than either can alone.