I don't think you have to replace them all at once, but if it has high mileage and since you are in there. My MC died earlier this year. I replaced it, it is hard to get in and out and it takes a while to bleed it. If you are leaking check to see if any is getting onto the clutch peddle, if so then it is probably the MC. When my fluid got low the peddle was real soft, if it got too low it would not work and I would have to refill it and bleed it.
Don't forget to
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first then ask questions later! The loose nut behind the wheel