I have a large bottle of Castrol break fluid that I opened about 2 months ago, so would this still be good, i.e. no significant moisture absorbed? I know it is not good to use old fluid, but a couple of months doesn't seem "old".
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You can still use it, but..... Once opened, brake fluid of any type (except silicone) begins sucking moisture out of thin air. This combines with the fluid to lower its boiling point. Incidently, since master cylinder caps are vented to the atmosphere, it is happening inside the brake/clutch system, too. And at some point, the built-up water separates from the fluid and settles to the low point in the system. It can even freeze, cutting off the brakes! If you're not competing nor driving real hard so the rotors are glowing, it will make little difference for a while. Ultimate life of the fluid will depend more on where you live (damp England being worse than dry hot Arizona). But its always better to buy brake fluid in small rather than lg cans due to the moisture increasing constantly upon opening. Brake & clutch fluid ideally should be drained & replaced every 12 months for corrosion prevention.