The gasket should not have limited life span. Good technique to check for leaks is to use piece of rubber tubing, hold it to your ear and use other end to listen around edges of manifold for vacuum hiss engine has to run for this procedure. You will hear all kinds of noises but vacuum leak is very distinctive.
It sounds like you took it apart already maybe inspect old gasket watch for irregular surface appearance, inspect manifold and heads surfaces too.

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