Brakes: Brake fluid - test or replace?

[Clan]

Originally Posted by Othen

Brake fluid doesn’t wear out, it just absorbs water if exposed. The only place it is exposed is at the master cylinder, so if you test it and it is okay then there is no need whatsoever to change it.

The business of changing brake fluid every 2 years or so is just made up by garages to invent business. I test it myself in all my cars (including my Porsche) and only change it if the tester indicates a need.

Alan

Its laid out in the volvo manufacturers servicing schedule actually and has been for some decades ...

[cheshired5]

The notion that good fluid in the reservoir means good fluid in the calipers is crazy.

I don't think changing every two years is necessary but use cases vary so there's no hard and fast rule.
It's roughly every 3 for me and I do the clutch at the same time for the same reason of not waiting for a race to the bottom in terms of fluid quality.
Testing fluid in the reservoir and doing nothing until it fails makes no sense though.

Engine oil is much more expensive yet we don't test that. We change it at sensible intervals regardless and brake fluid should be no different.

[DaveNP]

Originally Posted by Othen

As long as you fasten the half bottles with a secure top they will be fine.

Let's think about it: the water vapour content of air by mass is about 0.01% in your cold garage, air weighs about 1.5kg/m^3, and there is perhaps 500cc of air in your half used bottle, so there is 0.005*1.5*0.0001kg = 0.00000075kg of water in the air inside the bottle or 0.75mg. The remaining brake fluid weighs about 500g, so even if it absorbed all the a water vapour (and it cannot of course because of the partial pressures) then that would only be 0.00075/500=0.0000015 or 0.00015% contamination.

:-)

That's an extremely well reasoned argument and I can't fault it, but next time I'm going to do the brake fluid I'll still look at the half bottle and say 'A new one's only a tenner, let's get some fresh'

Originally Posted by cheshired5

...
Engine oil is much more expensive yet we don't test that. We change it at sensible intervals regardless and brake fluid should be no different.

We change engine oil either annually (it's summer - must be time to change the oil) or according to mileage which is displayed in front of us every time we drive; trying to remember was it last year? or the year before? or Oh ##! it was three years ago, probably doesn't help when it comes to disciplined changing of brake fluids.

[Derek UK]

Decant your half bottles into a smaller bottle. Combine half bottles or just buy smaller bottles when needed. If you pour fluid from one bottle to another or drop a bottle, allow to stand for at least 24 hours to allow the micro bubbles of air to settle out before use.

[Whippy]

I change mine usually at the two year mark. Why not?

[Tannaton]

Did a fluid brake change on a Land Rover Freelander the other week - fluid in the reservoir looked very clean (it's now an MOT test item), however when I bled the brakes, especially the rears, the fluid coming out was anything but, including some clumps of slime and white milky junk. The owner now reports the brakes feel much sharper - I did the clutch as well and that feels better too.

Don't forget on many cars with ABS and electric pumps for brake pressure (rather than vacuum servo assist) which includes many 80's/90's Volvo and most cars with hill start/auto hold there is a lot of brake fluid held in the accumulator on the valve block, probably as much if not more as in the reservoir.

So yes I'd advocate changing every 2 years, regardless of mileage.

[Othen]

Motor car and motorcycle maintenance is often such an emotive subject, there are always lots of views and sometimes people are downright rude to those with opinions not the same as their own - ho hum.

It would appear that most people disagree with me that brake fluid should be checked for water content regularly (annually) and changed as required. Well, here is a little anecdote to support my view: the only problem I have had with brake fluid in a motor car or motorcycle was after I had it changed by a garage according to the service schedule, the background is here (and in a few subsequent posts) should anyone be interested:

https://www.volvoforums.org.uk/showp...postcount=1574

... the fluid had been changed just 18 months to my Skoda's annual service in 2020 but the stuff in the reservoir was grossly contaminated (but that in the lines and slaves was okay). I couldn't determine how the contamination occurred (and it has not happened since, so we can discount a fault with the motor car) but if it had not been tested and if I'd just changed it after another 18 months according to the schedule then I'd have been driving round with a braking system full of contaminated fluid for all that time.

Since then I use my own tester annually (before they are serviced either by garages or by myself) on my 3 cars and 3 of my motorcycles (the other one has cable operated brakes) and generally change the fluid myself whenever it needs doing - that could be after one year (although that has only happened with the Skoda) or it could be after 10 (which it was with one of my very low mileage motorcycles). Brake fluid doesn't wear out (why would it?) it just gets contaminated, that depends on the use case and not particularly on time.

I have no view whatsoever on what other people spend their money on - they may change their brake fluid monthly for all I care. I still consider my testing routine to be most sensible and I'm happy to recommend to the OP that he does something similar (but I will not be offended in any way if he chooses not to).

[Othen]

Originally Posted by DaveNP

That's an extremely well reasoned argument and I can't fault it, but next time I'm going to do the brake fluid I'll still look at the half bottle and say 'A new one's only a tenner, let's get some fresh'

Well, the next time I need a spoonful of fluid to top up my brakes due to normal use (so the pads wear down and the pistons move out) I will reach for the perfectly good half tin on the shelf... but are you saying you would buy another litre and then have to dispose of (responsibly of course) 970ml?

I have no opinion on what you spend your money on David, but I don't like waste so I'll carry on storing half tins of DOT5 in my garage :-)

Good fortune,

Alan.

[andy_d]

If you're new to brake fluid, the terminology of a wet boiling point and a dry boiling point may be confusing to you. When the DOT rating refers to dry boiling point it means brand new brake fluid right out of the container. Brake fluid becomes wet when it's been in the lines for a while and has managed to absorb 3.7% water by volume. It takes about two years for any DOT brake fluid to reach the point where it has absorbed that much water. At that point you are going to want to have it replaced by way of a brake fluid flush.



If you allow your brake fluid to continue absorbing water beyond 3.7% by volume, which it will do if it doesn't get changed, the boiling point will continue to decrease. After reaching about 8% by volume, the boiling temperature will only be around 100 degrees Celsius.



The lower the boiling point of your brake fluid, the less effective it will be. Boiling brake fluid creates air pockets which diminish or completely eliminate the ability of your brakes to actually stop your car from moving. If it gets bad enough, you'll find that you are unable to stop in an emergency which could lead to some serious accidents and even death. That's why knowing the DOT rating of your brake fluid is important, as well as knowing when it was changed last and when it needs to be changed again.

which to be fair refers More to dot3/dot4 fluids as they are both glycol ether-based brake fluids.



DOT 5 brake fluid is a silicon-based fluid, which whilst doing the same job , albeit with a much higher "dry" boiling temp , as its silicon based it absorbs no moisture whatsoever, unlike the glycol-based fluids.
thusly rendering the "change at X years" nearly irrelevant for certain applications.
looks like Dot5 ,tho slightly more expensive could be "the answer for all", not so, as your cars brake system May not be compatible with it, especially if it has ABS ,ohh hello volvo, as there Are cases where the abs unit simply wont function correctly with dot5 in. there is also the issue of certain types of rubber seals used in the brakes that Could/Would be degraded by the silicon based dot5 and "hello brake failure" due to leaking seals,

additionally

When DOT5 brake fluid is used in an ABS system it will become foamy and unreliable. This is a result of the pulse high-pressure action that ABS brake systems exert on the brake fluid. The brakes will become spongy when you try to use them because of this as the foaming action will produce air bubbles that greatly reduces the reliability of the brake fluid. This is the main reason that you need to be 100% sure you're using the right brake fluid in your vehicle. If you have ABS brakes and you use the silicon-based fluid, you could potentially ruin the entire braking system for your vehicle. That could lead to a serious accident because your brakes are no longer able to perform the way that you need them to.

being worse case on the incompatible abs system. Dot 5 is also lighter than water, so if water Does enter the system whilst the dot5 fluid itself will not absorb the water, you can get water pooling at the lowest point of the system , hopefully the calipers so a "quick" bleed of the whole system before driving after a period of no use Should see you right.
Dot5 is also "paintwork safe" (mostly) so when spilled it Wont react with the paint.


Dot5.1 is a glycol based DOT 4 fluid that meets DOT 5 silicone-based fluid standards. The 5.1 fluids are used primarily in vehicles equipped with ABS brake systems.
so the Same hydroscopic nature, similar replace after X years , due to natural absorption, the same paintwork issues all apply, you just have a slightly higher "dry" boiling point,



how long Will it take to get the "magic number" of 3.7% ?
depends on
How often the car is used,
How the brakes are used ,
How good All the seals are
What conditions the car is used in (hot/dry/wet/going through water etc)
how long the Big tub sat open at whatever garage / dealer last topped it up And the temps/humidity @ said garage/dealership where it sat when the top was left off (again)


mix all of that with Volvo's ethos of safety and they reply with a Considered "replace after X years/ Y miles" which Will be on the side of caution , taking into account All of the worse case scenarios possible, so there corporate &rse can't be sued when "johnny no maintenance" has brake failure due to contaminated fluid and wants to sue the manuf as "well they said sealed for life" :P

[DaveNP]

Originally Posted by Othen

... but are you saying you would buy another litre and then have to dispose of (responsibly of course) 970ml?

Good fortune,

Alan.

Just to prove how illogical I am on this, for a top up I'd use the bottle in the garage, and my only defence is I can't remember when I last actually needed to do a small top up.