How long do you trust open brake fluid to still be good ?

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I already have fresh dot3/4 but I also have two containers that are half or more full. One is at least a year or more old the other is maybe 5 months or so. Neither was opened after the initial topping off and they kind of accumulated. So, use them for cutting oil or try them in the brake system of my 87 'burb ?

 

[mcbane]

I already have fresh dot3/4 but I also have two containers that are half or more full. One is at least a year or more old the other is maybe 5 months or so. Neither was opened after the initial topping off and they kind of accumulated. So, use them for cutting oil or try them in the brake system of my 87 'burb ?

Don’t know if it’s a rule but brake fluid turns brown as it adsorbs moisture. New/good fluid is very pale yellow.



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[Utahtrapper]

As long as the cap was replaced and very tight it should be good for a year or more.
If the cap is loose the break fluid will constantly be sucking moisture out of the air if you live with high humidity Florida, Midwest it does not take long for brake fluid to absorb moisture. Arizona and high Desert Utah very little humidity.

 

[ItsNemo]

You can pick up a brake fluid tester on amazon for under 20 bucks...I'd just use that. It is a sealed bottle (no different than a sealed brake fluid reservoir in the end) so it's likely good but worth testing before putting it in the brake system of a vehicle.

 

[garagelogician]

With the old style metal cans, it would keep pretty well with a tight cap.

But with the plastic containers, even when it sits on the store shelf and unopened, it is pulling moisture through the plastic. I only by big containers if I'm doing brake work, and I toss whatever is left over. If I ever need to top up (which you shouldn't need to if you have no leaks), I just buy a small bottle.

 

[rlitman]

Don’t know if it’s a rule but brake fluid turns brown as it adsorbs moisture. New/good fluid is very pale yellow.

The brown is actually the water in the fluid rusting parts in the system.

If you mix clear brake fluid in the bottle with water, it will remain clear.

 

[Tonyuk]

2 - 3 years easily if you replace the cap nice and tight.

They're are people out there driving about in 10 year old cars that have never seen a brake fluid change, it'll be good.

As mentioned you can buy a tester cheap enough online if your concerned about the moisture content.

 

[rlitman]

With the old style metal cans, it would keep pretty well with a tight cap.

But with the plastic containers, even when it sits on the store shelf and unopened, it is pulling moisture through the plastic. I only by big containers if I'm doing brake work, and I toss whatever is left over. If I ever need to top up (which you shouldn't need to if you have no leaks), I just buy a small bottle.

While plastic can be semipermeable to water, the bottles are too thick to have this be an issue. If it were a problem, bottled water would slowly empty while sitting on the store shelves.

Where it is a problem is in REALLY thin plastic membranes. Basically, zip-loc bags will cause freezer burn (water is sublimated out of frozen foods), unless you use freezer bags, which are simply thicker than their non-freezer equivalents.

 

[Lassen Forge]

We had a rule in one shop, whatever was left over after we closed shop got 86'd. They bought cases of small bottles, and you weren't supposed to have more than one bottle open at a time.

I felt that was kind of ****, but we were so close to the coast that the fear was the air that got in the bottle had enough moisture to contaminate the brake fluid.
Me, personally, once the bottle gets opened, if there's anything left, I'll keep it for a few weeks... but by then I start to worry a bit (because as was said above, the plastic bottles don't seal like the old metal ones do...)

If in doubt - throw it out.

 

[matt_i]

Imo its pretty cheap to replace, never worth using old.

As far as cutting fluid, I would not recommend this...it eats regular enamel paint for breakfast....

 

[finn]

Depends on if you live in the desert or at the seashore.

 

[DC73]

I only buy big containers if I'm doing brake work, and I toss whatever is left over. If I ever need to top up (which you shouldn't need to if you have no leaks), I just buy a small bottle.

I use this method as well. Brake fluid is cheap enough that I just assume any opened containers have absorbed too much water.

DC

 

[PassnThru]

2 - 3 years easily if you replace the cap nice and tight.

They're are people out there driving about in 10 year old cars that have never seen a brake fluid change, it'll be good.

As mentioned you can buy a tester cheap enough online if your concerned about the moisture content.

If the internet hadn't been invented we wouldn't even see this discussion between old school mechanics.

 

[Dragfluid]

Go out into the middle of the yard. Pour some into a paper cup. Quickly dump in a couple tablespoons of salt.

Back away.

 

[pi_guy]

I bought a tester for % of water and it only takes a few days for it to become contaminated.

 

[58Yeoman]

Last year, I replaced all the brake cylinders on my 63 1/2 Ford Galaxie, bought in 2013. It sits most of the time in the unheated shop. I'd driven it a few times...brake cylinders completely stuck. I did the same to my 58 Chevy, same conditions. I'd redone the brakes on the Chevy in the early 90's, nothing since. From now on, I'm going to bleed the brakes every two years, whether they need it or not. Too much of a hassle.

 

[MikeF2316]

Brake fluid sits in your master cylinder reservoir that has a small vent hole in its cap. For years.
Assuming a sealed cap is on the stuff on your shelf, you can use it until it's gone.

 

[cory58]

I think the “brake fluid sits in your car for years and still works” comments are irrelevant to the question. The reason to flush and replace brake fluid is to put in FRESH fluid so you don’t have to worry about it for a couple more years.

Brake fluid starts absorbing water as soon as it’s opened. It’s cheaper than replacing any brake part. No-brainer to buy small bottles and throw away what’s left, even for a cheapskate like me.

Cory

 

[Angelfire]

In my neighborhood, probably around 73 years.....

 

[JD3020]

Personally i limit my brake fluid to 2 months, if i open a bottle and don't use it all i write the date on it and put it at the front of the shelf. I always use quart bottles, and end up running 2-3 quarts through the system whenever i do brakes on our trucks(f-250's up to f-450) and SUV, whatever it takes to get clean fluid coming out of every caliper.

 

[Old Man Roger]

I'm no expert, but I have some practical experience abusing brake fluid...lol

Repeatedly glowing brake rotors, boiling brake fluid to the point of brake fade, glazing pads, warping rotors.

To be honest, most people never even come close to pushing brake fluid to it's limits.

Using a 2 year old, used, but capped bottle of brake fluid, is not going to be a problem in a normally driven passenger car.

Even if you're pushing the fluid to it's limits, it would still work fine, but it would lower the boiling point of the fluid. So brake fade would happen a little sooner.

It takes a lot to boil brake fluid, but every time you boil the fluid, it seems like it takes less time to boil it again.

The best brake fluid I ever used was Motul RBF600. It's expensive and shouldn't be mixed with other brake fluids. Flushing is recommended when switching to Motul RBF600

 

[like2wheel]

Brake fluid sits in your master cylinder reservoir that has a small vent hole in its cap. For years.
Assuming a sealed cap is on the stuff on your shelf, you can use it until it's gone.

That vent hole in the cap is always on the back side of a bladder type seal that prevents the brake fluid from being exposed to the atmosphere.
Pretty much makes your point irrelevant.

 

[58Yeoman]

I'm no expert, but I have some practical experience abusing brake fluid...lol

Repeatedly glowing brake rotors, boiling brake fluid to the point of brake fade, glazing pads, warping rotors.

To be honest, most people never even come close to pushing brake fluid to it's limits.

Using a 2 year old, used, but capped bottle of brake fluid, is not going to be a problem in a normally driven passenger car.

Even if you're pushing the fluid to it's limits, it would still work fine, but it would lower the boiling point of the fluid. So brake fade would happen a little sooner.

It takes a lot to boil brake fluid, but every time you boil the fluid, it seems like it takes less time to boil it again.

The best brake fluid I ever used was Motul RBF600. It's expensive and shouldn't be mixed with other brake fluids. Flushing is recommended when switching to Motul RBF600

Now you've done it. Unknowing people will now start boiling their old brake fluid to boil off the water before they use it.

 

[Old Man Roger]

Doh!

 

[Bad Eye Bill]

As long as the container is capped tightly, keep it it until it's all used.

 

[Brian_WK]

That vent hole in the cap is always on the back side of a bladder type seal that prevents the brake fluid from being exposed to the atmosphere.
Pretty much makes your point irrelevant.

Subaru there is a slit in the rubber seal and a groove that leads to a vent. It ensures that the pressure in the resivore stays the same, but keeps it from leaking out.

Brian

 

[volvosrock]

I do flushes every 2-3 years. Always use ATE branded fluid in steel cans. 1-2 cans per car and then I keep whatever is left for next time, never had an issue.

As a side note, I used to alternate their Super blue and their sl6, it made flushes easy due to the color contrast...just found out we can’t get super blue (us)here anymore, the coloring doesn’t meet DOT specs or something.


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[PoorOwner]

What about unopened ones ? I found a metal can of Ate blue I have over 10 years old. Can I use it ?

 

[Farmall450]

While plastic can be semipermeable to water, the bottles are too thick to have this be an issue. If it were a problem, bottled water would slowly empty while sitting on the store shelves.

Where it is a problem is in REALLY thin plastic membranes. Basically, zip-loc bags will cause freezer burn (water is sublimated out of frozen foods), unless you use freezer bags, which are simply thicker than their non-freezer equivalents.

This ^

 

[Aberdale]

It seems to me that if the failure of brake fluid is the absorption of water, then placing brake fluid in the freezer then straining it through a filter would remove all of the water since it would turn to ice. I might try that just to see what happens.

As to brake fluid degradation, there's probably more going on than just water absorption.

 

[ForceFed70]

I think you guys are too conservative.

I'd say the average vehicle goes 10 years before it's first fluid change. Yes, yes, I know - "not my vehicle". If it'll last that long in the vehicle under those operating conditions, it'll last a long time in a sealed bottle on the shelf.

 

[PNWguy]

I bought a tester for % of water and it only takes a few days for it to become contaminated.

Just out of curiosity, how does it compare to the fluid in your master cylinder?

 

[Evilunclegrimace]

Here is what the manufacturer says;http://www.epicbleedsolutions.com/resources/faq/shelf-life-of-dot-mineral-oil-brake-fluid/

 

[rlitman]

It seems to me that if the failure of brake fluid is the absorption of water, then placing brake fluid in the freezer then straining it through a filter would remove all of the water since it would turn to ice. I might try that just to see what happens.

As to brake fluid degradation, there's probably more going on than just water absorption.

No. Try that with vodka. It won't change anything.

 

[Showkey]

A few things to consider:

Vehicle manufacturers vary but many recommended brake fluid flush at 4 years......and the brake system is vented to the atmosphere.

So maybe the fluid in a some what sealed bottle is good for 4 years ?

A bottle sitting on the shelf is not vented and if it’s been open squeeze the air out. So the only moisture available is what can breath through the plastic bottle. Maybe a squirt of dry nitrogen in the head space would prolong the shelf life ?

What’s the sell by date on a sealed new bottle and how long past the sell by date is it still good ?? ( I doubt there is sell by date and likely has a lot code only readable by the manufacturer)

As a side note there are several notable sources that say the color of brake fluid is not an accurate judgement point. The color debate is as old as the fluid itself.

 

[pi_guy]

Just out of curiosity, how does it compare to the fluid in your master cylinder?

We flush the system before the car goes out. We might bleed as needed from driver or data. Some depending on driver & issues might bleed before each session that the car is due to go out.

The tester has been used only to condemn open cans of fluid.


Back in the 80's we did a driving school for LEO and showed them what happens after 4 or 5 hard brakes to fluid. We introduced them to the Castrol brake fluid and must better stopping power when the brakes were hot.

 

[IBenDcars]

Go out into the middle of the yard. Pour some into a paper cup. Quickly dump in a couple tablespoons of salt.

Back away.

I googled mixing salt and brake fluid. Didn't find anything. What's it do??

 

[MikeF2316]

That vent hole in the cap is always on the back side of a bladder type seal that prevents the brake fluid from being exposed to the atmosphere.
Pretty much makes your point irrelevant.

Nope. No car I've ever owned has a bladder inside the cap. My motorcycles, yes. Both cars I currently own have a float switch as part of the cap. There's no way to design a bladder to take up the space occupied by the brake fluid that remains in the calipers as the pads wear.

 

[Old Man Roger]

Nope. No car I've ever owned has a bladder inside the cap. My motorcycles, yes. Both cars I currently own have a float switch as part of the cap. There's no way to design a bladder to take up the space occupied by the brake fluid that remains in the calipers as the pads wear.

Some cars also have a bladder.

 

[LS6 Tommy]

There's no way to design a bladder to take up the space occupied by the brake fluid that remains in the calipers as the pads wear.

Funny, Most all American cars did it for decades...

Tommy