Totally assembled for a test fit with arrows showing the direction of flow. In this direction air flow/pressure will move the marble or ball bearing against the spring tension allowing air flow. If pressure is reversed as in a back fire, then the marble or ball bearing will slam back against the PVC seat sealing off the back fire. If some ignition leaks the stainless steel wool should catch it. In the next picture you'll see an added measure against back fire added.

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