There was no inner seal
Updated
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The confusion
A liner inside a cap and an inner seal bonded to the container land are different things. The first is a gasket, the second is a membrane, and the rule for brake fluid asks for an inner seal impervious to the fluid.
And a tamper-proof feature as well
Something that is destroyed or substantially altered when the closure is first opened. A plain continuous-thread cap meets neither half of that, however tightly it is run down.
Why it matters for this product specifically
Brake fluid is hygroscopic. A package that breathes slowly takes up water, the wet boiling point falls, and the number printed on the label stops describing the contents. The seal is protecting a claim.
Induction sealing is a process, not a part
A foil-lined cap only becomes a seal after it passes under an induction head at the right power and line speed. Buying HIS-lined closures without the sealing step gets you a liner and no membrane.
And the land has to be right
The sealing surface on the container has to be clean, flat and free of flash, or the bond is partial. A partial bond looks sealed and leaks or breathes, which is the worst of both.
Check the liner field, then ask the supplier
Of the 169 closures on this shelf, 67 carry an HIS liner and 28 carry a pressure-sensitive one. The field is published on every closure page; whether a specific pairing passes a standard is the supplier’s test report, not our data.
Test a filled unit over time
Sealed, stored, and the contents checked after months rather than days. Moisture uptake is slow, which is exactly why this failure reaches customers instead of being caught on the line.