It's the flat washer that 'seals' the tank. The o-rings serve a different purpose. Any 'rubber' ring will take up a temporary deformation, but it's still working. Only when the 'rubber' loses it's elasticity (for want of a better word) through age and heat degradation will the seal become weakened. That's called compression 'set' and is irreversible. Some 'rubbers' deteriorate faster than others. The best for this application is Viton. All rubber washers and o-rings need a lubricant. Few header caps get that.
TC
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