I agree with Jim, the tank has to be vented. The original method was through the cap.
If the caps today are not freely venting they need to be modified. Or find a 60 year old NOS cap if you don't like the idea of modifying one.


1951 GMC 250 in the Project Journals
1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-