As I said, if DOT 4 has been in there for a while, you often can't avoid replacing at least the rubber hoses.
And yes - that's not uncommon, because on the one hand, people often don't read the
manual , and on the other hand, they often listen to workshops when they say, "Here, use DOT 4, we use that for all tractors..."
And don't forget to apply plenty of "brake cylinder paste" (from ATE eg) to the cylinders under their dust caps; this is the only way to keep rust away from the brake cylinders for a very long time.
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And please don't confuse this with "brake paste" (usually a ceramic paste) or "copper paste" - they're different things.