Aqua-Magic V

Had the same problem and replaced the fill valve assembly with one off fleabay link. A heck of a lot cheaper than a new toilet and since the rest of the commode is in excellent condition it's win-win. Oh and less messy than removing the seal on the bottom of the toilet.

I've changed Thetford water valves and had it turn out OK. We have the Aqua Magic IV, which is about the cheapest-looking toilet you could imagine. Yet, it has been absolutely fault-free.

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Had that happen numerous times. I've always found it easier (and less messy) just to replace the toilet. Replaced a few water valves in the toilet but they don't last. Best replacement is usually the simplest (cheapest).