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That's a symptom of a bad check valve, which is easy to test.
Remove your check valve, put the port that is normally connected to the intake manifold through the filter to your lips and try to blow through it. If you CAN blow through it, the check valve is bad and it should be replaced.
I never know on my 73 which one's coming up first. Me and my buddies get together and after a few beers we place bets on which one. It's always around a 50/50 chance!!
That's a symptom of a bad check valve, which is easy to test.
Remove your check valve, put the port that is normally connected to the intake manifold through the filter to your lips and try to blow through it. If you CAN blow through it, the check valve is bad and it should be replaced.