Brake fluid - how long until unusable/does anyone North-West-ish want some?

After doing my brakes recently I've been left with a fair bit of this
left. As I've not done this before, how long until it becomes unusable
now I've opened the pot?

If it's still ok at the moment, does anyone want it/their brakes doing?

[1] i.e. a few weeks since initial opening, with the top screwed on as
tightly as I can get it since.

 

Seal it tightly, and it lasts for years, as long as you don't shake the
bottle (frothing and air buibbles, and even then it'll be OK if you let
it settle).

Well, I've used elderly well-sealed brake fluid and I'm still here to
tell the tale.

Seriously, the only real worry is the ingress of moisture as brake fluid
is highly hygroscopic. But sealing the container properly will take care
of that.

 

The bottle says to use a fresh one every time.

However, I have a bottle I keep in the garage for topping up shite old
cars as and when they need it.

I'm sure you're not supposed to do that - but I've not had a problem
using stuff that's as much as 6 months old.

 

Six months?

Wimp!!

 

You'll note my reference to Shite old Cars, and realise why I only have
it for 6 months ;-)

 

the cont known as The Older Gentleman says...

Hadn't that already been established?

 

Cheers to you and TOG then - it should be fine in the bottle it came in
as it's not squisy with the top on, indicating it's pretty airtight.

 

The reason it becomes unusable is because it is hygroscopic (absorbs
moisture from the atmosphere). If the air space in a tightly sealed
container is relatively small then there is not likely to be any appreciable
risk. However, a small amount in the bottom of a large container would pose
more of a risk as there would be more air above the fluid from which water
might be absorbed. If the container is not tightly sealed then there would
be a considerable risk that would increase the longer the fluid was unused.

Tony

 

Afterthought - the above only applies to glycol based brake fluids -
silicone based brake fluids (DOT 5) do not have this problem - however, they
tend to make seals that contain any natural rubber swell much more than
glycol fluids which can lead to leaks - and silicone fluids are more prone
to foaming and also become compressible at temperatures near to their
boiling point which may make them unsuitable for high performance vehicles
or racing vehicles.

Tony

 

Cheers matey - so it's just pop the egg in and do what?

However, this is new to me. I thought all brake fluid was hygroscopic,
not just DOT 5, hence the "only use new, from a sealed container"
warnings.

 

You have read it wrong, Chris. The silicone fluid is not hygroscopic,
but all the rest are. You don't want to use the silicone variety
though, as it compresses and boils more easily, plus you have to flush
it thoroughly through the system if replacing conventional fluid, as
the twain must never meet.

DOT5 was a lovely idea - non-hygroscopic, non-corrosive (paint
friendly) but a bit of a non-starter in the functionality stakes.

As far as the longevity of fluid goes - I've used sealed bottles that
have been on the shelf for *yonks* and it has been fine. I've also
used fluid from opened containers that has been on the shelf for yonks
(like two years plus) and it has been fine.

I've even used the gradually diminishing remnants of a 5 litre can of
the stuff that lasted me about four years, and it was fine. Just
don't shake before use, that's the only rule I acknowledge - oh, apart
from washing it off painted surfaces pdq.

 

Well, that's pretty much how my knowledge of brake fluid stood before
this thread, the only bit missing being experience of using fluid that
has been open for x days/months/years.

Apologies to TMack then, for reading his post wrongly.