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As mentioned by Barry earlier, check that the bowl itself is not leaking.
The brass overflow tubes can become split from freezing or not seal well to the bowl.
Fill the bowl with fuel on the workbench and check for leaks.
Apologies if you have already checked this.
Brett

Tobster,

The "locked" state of these caps is when they spin freely.

Typically when they are unlocked and fail they will give a ratcheting feel as you turn them.

I would advise putting lots of lock lube in the keyway and working the key in and out in hopes of getting to the unlocked state.

Best ...