I know that this stuff is hydrophilic and so has a shelf life once opened – but how old is too old?

I have to bleed my brakes at the weekend and the fluid has been open for I’d say 18 months or so.

I chuck any stuff over 18 months old if it’s been opened, but I’m finiky about stuff like that, will probably be fine.

Some people say chuck it away if it’s been opened, don’t use it again. Even if it hasn’t been opened the moisture gets through the bottle walls eventually.

If the seals broken I wont use anything opened over 6 months ago. I leave it in the bike for 12 months, which would then means its been able to soak up moisture for about a 18 months and you are meant to renew it at 2 years anyway. Probably a bit wasteful mind 😳 I only use the little bottles from Halfrauds though so doesn’t get too much wastage from a few set of brakes.

I’ve got an 8 year old bottle and not died.

There’s a breather hole in the lid of most car brake resevoirs, if it can last 2 years in there wihout absorbing too much water, it’ll survive 18month in a bottle with a closed lid. Once it’s dessicated the air in the bottle theres no more moisture to absorb, and the wet boiling point is measured with a suprising ammount of water anyway so the tiny ammount of moist air that got in last time you used it wont affect it.

You replace it in your bike as the seals in the calliper aren’t great when not under pressure, so when you release the lever they’re drawing water in. Which is why 2 year old brake fluid in your brakes is dirty and cloudy, and 2 year old stuff on the shelf is still clear.