The diaphragm is shown as Item 394 in the Illustrated Parts List, Paul, and it is shown in two pieces. The one that goes against the carburetor acts as a gasket, and the one that goes against the top of the fuel tank acts as the diaphragm - it seems Briggs now only sell the two parts together rather than separately.
If you look at the diaphragm part, you will see the area that has to stretch up and down as it pumps (the diaphragm function), and two flaps of the rubbery material that have to open and close holes in the top of the fuel tank (the inlet and outlet valve functions). Diaphragms usually fail because the flaps no longer lie flat and seal the holes in the tank, or less often, the part that pumps by stretching up and down has gone a bit stiff or baggy. You may be able to see a fault in one of those areas by looking at it closely. Sometimes you can't see a fault, but the problem still goes away when you replace it.
