Moisture in brake systems?

Moisture in the brake fluid will move though the whole system. If the moisture level is high enough it will cause the internal corrosion you have found. With high enough moisture content the water will separate out in the wheel cylinders.

Any car sold 2nd hand out of makers warranty is likely to be in urgent need of brake fluid, coolant and thermostat change even if it has a book full of dealer service stamps.

These days brake pads are changed by just pushing the piston back in and releasing the old fluid. When doing front pads the rears won't be touched. Just enough new fluid will be added to show clean fluid in the brake reservoir, you still get charged for 1L of fluid. Rear drum brakes, the shoes last well over 60K miles and the fluid never gets bled from the wheel cylinders until it needs new wheel cylinders.

From your own experience you have seen how moisture DOES get into the system, so why are you questioning the need for fluid replacement?

Fresh brake fluid can and will absorb a certain amount of water, not just at the master but through rubber hoses and seals, once it does, the boiling point is reduced and corrosion will occur when the water content is significant.

I change brake fluid whenever I replace brake parts OR a test shows too high a level of water in the fluid. Most people get through front pads in about three to four years and it is rare that a test of the fluid before that will show significant moisture.

Many people do not realise that once a bottle of fluid is open it starts to suck in moisture and should be discarded after a year or so. Another problem I used to see a lot in the motorcycle shop was people shaking brake fluid before topping up, that entrains tiny bubbles of air into the fluid.

My personal experience says regular fluid changes greatly extends brake component life. But you have to trade off the cost of this against the cost of replacing parts where necessary. And starting regular changes on an old car which hasn't had them before may not be worth it - unless you'd just fitted all new parts.

Understood.

So that is the case then, the moisture level in the fluid *will* cause corrosion but at what percentage is it likely to be at a rate that exceeds the regular wear / failure rate?

Ok. That is was I thought it be the case, judging by the amount of rust I've found *inside* a cylinder that must have been like that for some time before it started leaking.

Agreed.

Oh, no retaining pins you mean (just castings in the caliper or some such)?

I can believe that. A mate used to do loads of ex (supposedly) dealer-serviced cars are few of them had any signs of the rear drums ever being removed.

Nice.

Well, I know an opened brake fluid container is likely to attract moisture and so I think I might not mind being charged for 1l if they have used only 500ml. However, I can't see why I couldn't be given the remaining fluid (although I'm guessing few would want it if offered).

Understood.

So, do you or would you use one of the electronic moisture detector pens Peter and if so, at what point would you consider it appropriate to change the fluid (assuming you don't know it's history)?

Cheers, T i m

I think a fiver's worth of fluid on an unknown car is money well spent (even if you get a garage to change the fluid it is only around thirty quid all in.)

Well, I was just checking that any water in the fluid that *might* cause corrosion *was* being drawn in and held because of the hygroscopic nature of the fluid, and not some reason I wasn't aware of?

See above. The question came about in my mind because someone who had been maintaining other peoples vehicles for ~45 years suggested that the moisture level detectors and subsequent corrosion was a non-issue and often used as just a bogus means to make money.

From further questioning though it may have been that if he found a leaking cylinder, because he was doing it *professionally* he didn't always strip the cylinder / caliper down and investigate the cause. I on the other hand and especially when just an impoverished student, would strip, clean, hone and re-rubber most bake actuators and with no issue. Whilst doing so I often observed current or possibly the signs of previous corrosion, hence the need for a 'second opinion'.

This was more relevant on motorcycle brake parts as they were generally much more expensive.

Ok (thanks).

Ok. I would say I generally don't (reluctance to open up another can of worms [1] with seized bleed nipples or systems that won't bleed easily), unless we are talking opening the hydraulic system then I would also do so (eg, bit not just for pads or shoes etc).

JOOI, what sort of readings do you get and what is your threshold for 'too high'?

Oh, ok, that's interesting (and one of my questions).

Agreed.

Yup, and funnily that's what my mate (owner / mechanic) at the local bike shop said just yesterday. ;-)

Ok, we know the air bubbles will slowly settle out but you don't want to make the job of bleeding the brakes (and looking for a 'firm pedal / lever') any more difficult (or needing repetition).

Cheers, T i m

[1] Depending on the need for availability of the vehicle by it's owner etc. If it's 'a project' then I would always change the fluid and bleed the entire system (this happened when we changed the slave cylinders on my mates Landy the other day and he had to get new hoses and pipes because they were corroded / seized or perished).

Makes sense.

True.

As we did with mates Landy the other day.

So, do you have / use a moisture content device Dave and if so, what sort of readings can it give?

I think the bottom line is that most would agree that you don't want

*any* moisture in your brake fluid but given that you will probably always read some, at what point is it worth replacing the fluid?

eg, The electronic thing I have here suggests anything 3% and over is 'Warning' but what if that's way below the threshold where corrosion or boiling might take place? Like what if it has to be 10%+ moisture for that?

Cheers, T i m

Many older mechanics cannot or will not move with the times.

Wheel cylinders and indeed most service parts are now so cheap that no-one would bother to take a leaking cylinder apart any longer, I have not honed and resealed a slave cylinder for at least the last thirty years

You should, pushing dirty old fluid back into the most sensitive parts of the system (master cyl, abs etc.) is a must-do nowadays.

at 2 % I put it on the bill as 'change at next service' 3% I change it regardless, I have not yet seen a 4%

Oh. indeed. But not sure it will be quite the preventative maintenance as those done from when the car was new.

I don't think there is any threshold below which corrosion won't take place, it's just how much. One molecule of water will corrode one molecule of metal, it just has to reach the metal and not be floating around in the centre of the fluid. I think that's why there is a percentage level, the more moisture the higher the chance of it meeting the metal walls.

It would be interesting to know what the moisture level was at the brake cylinder compared to in the reservoir since any moisture will slowly make it's way to the lowest point.

Didn't bother. Just changed it every two years regardless of use.

I agree that's probably the case in many cases but this one isn't quite like that. He recently bought a new diagnostic unit, has aircon machine and is a private pilot. He even has a smart phone. ;-)

Quite, but the suggestion was that moisture never caused a problem, not that he hadn't witnessed any signs of any himself.

Understood.

Oh ok, so the same sort of ranges I have on mine.

We did the other day on the rear brake of daughters 600 Bandit! ;-(

Mind you, it didn't look like any of it had been touched for *years* and you have to remove the rear fairing (and to do that the rear grab handles) to get to the reservoir.

At least the front is easy to get at as it's up on the handlebars. ;-)

Cheers, T i m

Makes sense. I had assumed though that you needed 'air' for corrosion on an otherwise sealed from the atmosphere system but as water is H2O ... ;-)

Understood.

Makes sense.

Unless either was between the 0-4 range my gadget wouldn't be able to differentiate and as the fluid in daughters rear brake reservoir was over 4%, I'm guessing the fluid in the caliper would be worse. ;-(

We have several vehicles so I will try to test those as you suggest (reservoir - cylinder / caliper).

Cheers, T i m

How very organised. ;-)

Cheers, T i m

The above should of course read MUST NOT DO.

Letting fluid out at the wheel is a MUST DO

(I was putting too much into one sentence and did not check)

Yep. OOI, why "intended to be"?

Rarely, I reckon, but both our cars get it done.

It didn't need any correction for me mate. ;-)

Cheers, T i m

Because I understood brake fluid is *intended* to be hygroscopic to stop the water separating out and causing other issues (spongy / no brakes). I thought that was one of the reasons that silicone brake fluid isn't recommended for ordinary folk because it *isn't* hygroscopic.

Noted.

Cheers, T i m

This seems to explain why that might be a good idea (not suggesting it wasn't etc). ;-)

"Today, brake fluid can be accurately tested to determine replacement needs. This is because the most common issues stem from the levels of dissolved copper and depleted additive package in modern brake fluids. The level of dissolved copper in brake fluid is an indicator of the health of the brake fluid?s additive package. When the additive package of brake fluid is depleted one of the negative results may be internal brake system component corrosion and or sludge build up."

So, it seems whilst the chances of getting water into the system is less these days, the fluid can still break down (over time) and be less able to protect the braking internals as well.

Also, it mentions when that happens there will also be a buildup of copper and I wonder if it's also that increased conductivity these electronic meters are measuring?

Not that it matters as such as water or excess copper suggest the same thing, that you should change the fluid. ;-)

Cheers, T i m