Brake Fluid..

**macdad** · 12th July 2008, 18:51

??Does brake fluid have a shelf life so to speak,doing a fix on the wifes car(rear brake flexi hoses) mot failure,obviously need brake fluid for job and well noticed 500 ml bottle in shed which i'd completely forgotton about,only thing is i bought the stuff about 3 years ago...what i want to know is?can it still be used safely...any doubts and i'll gob it.Cheeers lads.

**glynn** · 12th July 2008, 18:54

the AA recommend its changed every 2 years as i breaks down through heat and condonsation can also brake it down

**glynn** · 12th July 2008, 18:56

is it a brand new bottle and if so it should still be ok as you are only topping the fluid up

**abbeytec** · 12th July 2008, 19:20

Brake fluid should have a proper sealed top (under the screw cap).

If that seal is unbroken, it will be fine. If its broken, bin it.

Brake fluid is HYGROSCOPIC, meaning it will attract (and absorb) moisture from the atmosphere. Water in the brake fluid will lower its boiling point, which can make it boil under hard usage. This causes vapour in the fluid, which is a Very Bad Thing...

**ed taylor** · 12th July 2008, 19:22

I don,t know of a shelf life. But it is Hydroscopic (it absorbs moisture) so the longer it is on the shelf the more moisture it would absorb. Me I would be more inclined to buy a new bottle ( asuming it would be newer fluid than the one you replaced) as your brakes are quite an important part of the car.
PS abbytec. you were a bit quicker with your reply than me.

But the sentiments are the same.

**macdad** · 12th July 2008, 19:41

The plastic bottle is sealed and has never been used in which case moisture i would have thought can't get in there...i'm tempted to use it.

**abbeytec** · 13th July 2008, 10:31

Yep, agree with you Ed, except that you are under a common misconception that there is such a word as Hydroscopic, but there isn't... Try Googleing it. (Is Googleing a word???)!!!

The word is (as I said) HyGROscopic

Yeah, yeah, I know - Pedantic G1t

**Ste** · 14th July 2008, 08:31

I would use it anyway.

At the end of the day, the bottle is air tight, so how can air and therefore water vapour get into the fluid????

Horses for courses, but I've never ever changed my brake fluid in any car I have ever owned, as a routine maintenace activity.

Yes, I understand that brake fluid absorbs water, BUT, can someone explain to me (using simple language cos i'm only an automotive Engineer) how does the vapour get into the fluid at the business end (ie at the brake surfaces) ? ? As this is where it gets hot and bothered.

**abbeytec** · 15th July 2008, 22:46

Originally Posted by Ste

I would use it anyway.

At the end of the day, the bottle is air tight, so how can air and therefore water vapour get into the fluid????

Horses for courses, but I've never ever changed my brake fluid in any car I have ever owned, as a routine maintenace activity.

Yes, I understand that brake fluid absorbs water, BUT, can someone explain to me (using simple language cos i'm only an automotive Engineer) how does the vapour get into the fluid at the business end (ie at the brake surfaces) ? ? As this is where it gets hot and bothered.

Well I would GUESS, (note the word guess), that its because it is not a fully closed system - air MUST be able to get into the reservoir, or an air-lock would develop as the pads wore down, and fluid was drawn into the pressurised part of the system to "take up the slack" as it were.

With the fluid being so agressively hygroscopic, I imagine that any moisture will permeate throughout the fluid, and therefore affect its properties.

**Ste** · 16th July 2008, 11:40

I don't subscribe to changing brake fluid routinely as a maintenance activity. IMO it's just another way to extract cash from you and is unnecessary in 99% of cases.

The question about air being drawn into the res and then the water miraculously being forced up the brake pipes by osmosis (or whatever action). The air volume being drawn in is low, and if the water content is a fraction of this, how much water actually gets into the brake system and then travels to the business end?? Probably not a measureable amount.