All 4 of my vents are rusted and seized. After using penetrating oil for 3 days and a hammer and punch they still did not move.

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So I removed all 4. Only 4 screws out of 10 were holding each in place. All came out except 2 on one vent so I cut the heads off with a dremel tool.

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Bought some acid to clean them up.

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After only a couple hours looking better.

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Still seized however. So will soak some more and hope for the best. As a last resort I will remove the pins holding the latch parts together and replace with machine screws. The spring inside may need replacing anyhow. We shall see.