My percolator stopped perking.

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I bought a vinatage 1950s coffee-matic percolator from ebay. everyone raves about it and so have I. But a couple of days ago, it started percolating about 20 seconds before the ready light goes on. The water is clear, but very hot. All I have to do is pour the water in the basket and we have coffee. But does anyone know what's wrong? I'm thinking it's something simple.