'58 VW Bus Gas Tank Leak

It could be that the wrong gas cap is fitted. The old VW gas tanks don’t vent. Each time I undid the gas cap to refuel my '66 camper it was like Old Faithfull erupting. The fumes whoosed out and I had to hold the cap tightly to avoid it being propelled at me. I fitted a proper vented cap and did not have any more problems.