Moisture in brake systems?

Water diffuses through the rubber of hoses and seals and as the master cylinder is vented water can directly get at fluid there, so ideally fluid should be tested at the master cyl and at each wheel. In practice of course, it is only tested at the master.

OOI US military swapped all its vehicles over to silicone fluid in the early 90's, silicone does not absorb water and so may be left in without worry or corrosion indefinitely.

Every time the brakes are used the fluid level in the reservoir drops and then rises when the pedal is released. This moves fresh wet air in and out of the reservoir. This source of moisture has got to be far worse than osmosis though a rubber hose.

Motorcycle master cylinder reservoirs don't have fluid in contact with fresh wet air. They have a rubber bellows diaphragm under the cap. That's got to slow down the rate of brake fluid moisture pickup on motorcycles considerably.

Unfortunately it does not upscale to cars easily. Testing at the reservoir is likely to show higher levels of water I agree, but both ends of the system are at risk.

I do an oil change based on time since the annual mileage on the old car is low. So just change the brake fluid every other one.

I bought a little pen shaped 'stick' with several LEDS and two probes on the end you dip into the fluid and it shows the reading on the LED (0% to 4%).

This is apparently less of an issue these days (osmosis though the rubber).

As answered previously the only place it typically gets measured is at the mater cylinder but it's also been mentioned that any moisture is likely to gather at the lowest points (water having a higher specific gravity than brake fluid).

I an confirm I have had more brake cylinders and calipers showing signs of corrosion than I have master cylinders, except maybe on my Morris Minors when the master cylinder is under the driver feet. ;-)

Cheers, T i m

Agreed ... but daughters Bandit rear brake fluid sent my detector off the scale (and it looked 'discoloured') and that had the rubber diaphragm as you say, so the fluid could well have been in there for a long time?

That plus the pipe from the rear brake master cylinder to caliper was quite long and all rubber (it looked longer than a std car rubber hose but maybe not by a lot).

Cheers, T i m

That's my understanding too. Silicone oils are slightly less dense than water, so water will end up at the hot bits, just where you don't want it. Maybe the appeal to the military is that silicone is non-flammable.

Which begs the question as to why (distilled) water is not used as the brake system fluid !!!!! (Sorry!)

because it can boil at a much lower temperature.

but apart from that and the freezing apart from that as well and the corrosion (because it won't stay as pure water for long) So apart from the boiling, freezing and corrosion :-)

Any one reason is enough!!

But at least the Romans didn't invent it.

"Roman engineers developed an advanced expertise in hydraulic engineering."

(HYDRAULIC ENGINEERING IN ROMAN TIMES: FROM HYDRAULIC STRUCTURES TO WATER CONVEYANCE H. Chanson1 )

I am rightly corrected

T i m brought next idea :

The cheapest of those pen type fluid moisture meters I could find, was this one -

Yup, that's the badger and whilst I ordered my second batch from China, they were here in a week. ;-)

Cheers, T i m

Ah, yes. Makes sense now.

Many sellers with same item at a higher price and claim to be in London have registered Ebay addresses in "Shanghai China".

alan_m presented the following explanation :

I have come across that many times. A clue to where it might be sent from, is the estimated time for delivery. My experience suggests they seem to have or use a faster delivery system than is usual from China, or that they send from somewhere closer, like the EU. The package labels give few clues as to where they have actually been sent from.