Hi, are there any other advantages to changing the brake fluid other than because of excess water absorption? The reason I ask is that my 04 XK8 is up for a change this year; I was considering getting one of those cheap electronic brake fluid testers and seeing if it >really< needs to be done.
I've read that some of those testers are not particularly accurate.
Does anyone else only change it only when necessary or does replacing the fluid also help other parts of the brake hydraulic system?
Thanks.

 

Fluid replacement every two years is an Absolutely must to preserve the life of the Brake system..

I don't care what type of brake fluid is used..(we call it here in the US, DOT-3 or DOT-4)..they all have a certain amount of hygroscopic tendencies..(Affinity to absorb moisture) And in your part of the world, I'm guessing you have higher atmospheric moisture.

..The problem is that the moisture even though it is suspended in the fluid will eventually concentrate and collect at the lowest points in the system,(Brake calipers or cylinders, as water is heavier than the fluid) and this is where you need the braking action to take place obviously and you can't test the fluid there..The moisture will attack the seals and 'O'-rings and piston walls and cause corrosion, which will ultimately cause the caliper to hang up or fail...Bleeding of the system purges all this out.

Another aspect of severe moisture in the braking system is vaporization when you jam on the brakes..This causes the moisture to boil off into a vapor, creating a vapor bubble, which will give a affect of spongy brakes....This of course is an extreme worse case scenario.

This is a cheap maintenance task and shouldn't be neglected or short-cut..

 

+1

All the above plus:

When you open a bleed screw and see the dirty fluid that comes out, you'll be convinced.

It may look nice and clean in the reservoir but it's very different at the caliper.

Graham

 

Yes, primarily contamination from seal degradation that could act as an abrasive but the rate of that is so slow that it's not worth worry about.

I think if an accurate instrument measuring moisture content were used, the amount absorbed over the present two year change period would be found to be quite low also.

While OEM's have made great strides with extended engine oil change intervals, they're still stuck in the '60s when brake systems were vented to the atmosphere at the master cylinder. Modern systems are sealed, usually with a diaphragm in the M/C, limiting the exposure to moisture in the air.

I think BMW and a few others have extended the interval to three years, still very conservative.

 

Thanks guys esp. DPK for the down n' dirty details. I shall definitely be changing that fluid this year!

 

You're very welcome...

If you have a spouse to help pump the brake pedal..tell her to think of it as an aerobic exercise and will help firm up those flabby calves and thighs..

 

As you're in the UK, the Guyson Eezibleed Kit makes a straightforward job of flushing the brake fluid.



No pedal pumping involved and you can watch the clean fluid coming through at each bleed screw.

Graham

 

That's pretty cool

 

Guyson may be Gunson.

I like to use a VERY low PSI in the tyre - below 10 (about 0.5bar) - with modern ABS-type cars.

If pumping via the brake pedal try not to go all the way to fully depressed as that much travel has never been used.

+1 on at least bleeding out the worst stuff and once you're doing it at all you may as well bleed plenty through.

Some of the jags use Super DOT 4 and it sounds good stuff.

 

May I ask just more quick question? The old fluid is 2 years old, is there much of a colour difference between old and new (as you may have guessed I haven't done this job before!) Would I be able to clearly see when any particular leg of the system is completely flushed of the old stuff?
From what JagV8 says it sounds like it's best to flush the system to it's normal capacity and then more so to make sure.
Thanks.

 

YES - my typo.



Graham

 

Once the dirty fluid immediately next to the caliper has flushed out, you won't see much difference in the rest of the fluid coming down the line from clean, new fluid.

I usually buy a 1 litre container of fluid, use 90% for flushing and keep the last 10% for topping off after a brake test.

Graham

 

Sorry fellas , but I tend to disagree with some of whats been said about the urgency and methodical replacement of brake fluid .

"mulitiplecats"
Just how much moisture do you really think is in your brake fluid , when you do bleed or flush your system , you'll have to look pretty hard to see the amount of water you have actually collected .

If you want to change your brake fluid because you can see that it's dirty , then go ahead , but you won't get a spongy or low pedal because of it .

"DPK"
"Fluid replacement every two years is an Absolutely must to preserve the life of the Brake system" , what exactly are you preserving ? , caliper seals wear out normally ,
piston dust boots split and crack with heat normally , master cylinder seals and booster rubbers wear out normally , disc pads and rotors wear out normally and so on .

And you are right , brake fluid by it's very nature attracts moisture , so after 10 or so years you'll still have corrosion built up in the calipers , and you'll need a scraper to remove that , let alone a brake flush every 2 years .

"JagV8"
"If pumping via the brake pedal try not to go all the way to fully depressed"
Good point , and with all due respect , do you know how hard it is to do that , and not over shoot the mark , especially with little to no pressure at the pedal while bleeding .

It's been said many times on this well informed forum , that some people do encounter a low brake pedal or worse than before feeling or a failed master cylinder , after , bleeding or flushing their brake fluid .

My car , like some of the members out there , is over eight years old with no fluid change or ill effects to the brakes at all , and yes , I will eventually change the fluid .

And multiplecats , if you did want to step up on brake fluid , try DOT 5.1
it maintains fluid viscosity better at hotter temperatures .

Well , good luck with that anyway .

 

Agree with your comments about the frequency of flushing the fluid. Most of the discoloration of fluid in the calipers is carbon particles released from the seals and is essentially harmless. The two year interval applies to the old 'open' systems that have not been used since the '70s. Although the question has been asked before, no one has been able to describe the actual path for moisture to enter the system and contaminate the fluid. I've never seen anyone produce a sample of water-contaminated fluid from a modern car that's in otherwise good condition. Perhaps a review of the interval is in order so that Jag keeps up with other OEMs?

DOT5.1 does indeed offer some improvements over DOT4 or Super DOT4, but they're essentially wasted on street driven cars. The brake systems never get hot enough to take advantage no matter how hard the car is driven.

 

My buddy worked at the Rolls Royce dealer back in the 70s and his 'mentor' always threw a hammer under the brake pedal before he let someone 'pump-the-brakes' for bleeding.

He got tired of customers coming back a few days after brake work complaining of master cylinders 'bypassing' due to debris getting into the cup seals at the bottom of the cylinder. The piston does not ever get to the bottom of the cylinder under normal use. JUNK collects down there!!!!!!!!!!!!!!!!

You can control your foot if you think about what you are doing.

bob

 

And that would come from .... corrosion of the bore scoring the seal.

Corrosion that was caused by water in the brake fluid.

I would hazard a guess that a vehicle kept in a dry climate
needs less attention to water absorbtion than say someone
in the UK.

As for "sealed", umm ... it's pretty good but not perfect.
That imperfection is the problem.

 

Yes - water molecules are small and some get in.

I like the hammer idea (but currently bleed with the low pressure tyre/kit).

 

All good information guys, thanks again. Sealey do this gadget which appears to be like the Gunson but has it's own pump so you don't need the tyre etc.
Sealey

 

The SEALS of a brake fluid system are made from ACRYLONITRILE-BUTADIENE RUBBER (NBR). No carbon components to discolor the fluid..The darkening comes from aging and moisture.

 

"And that would come from .... corrosion of the bore scoring the seal.
Corrosion that was caused by water in the brake fluid."

Hmm... OK ,

Lets take the moisture and water causing corrosion out of the equation for a minute .

So , were are now free from any of the above contaminants , in your opinion , would it be now safe to bleed and flush the brake fluid ? .