Kind of interesting to note. When I rebuilt my '46 mechanical pump, it was the (under)side that does not come in direct contact with any gas that was deteriorated. I rebuilt it before it failed. The (top)side that comes in direct contact with gas appeared fine visually but the other side was cracked right up. This pump only tasted gas with ethanol in its later lifetime.
If you rebuild it, be sure to save and reuse the old spring.
(top)Side contacting gas directly.
(under)Side contacting crankcase vapor/fumes
Side contacting crankcase vapor/fumes
