Just a wee add on to the big post I made above. Quite a number of years ago I bought a quite expensive boiling point brake fluid tester - a really professional piece of kit. Why would I do that when I'm not now working professionally any more and just maintaining family vehicles? Well, at least 75% of these vehicles are quite old so tend to have corrosion problems and one of these is seized up bleed nipples on callipers and wheel cylinders - most of us will have had problems with these snapping off - eh lads? A pain in the butt which can turn a simple two minute job into a bit of a nightmare. So I decided a fluid tester might mean I would have to change fluid only when really needed - so less frequently. Well, the tool works really well, It's a Liquid Levers Suretest. which cost £139 back in 2013. I was lucky in that my three children payed for most of it as a Christmas present, I had to make a small top up addition. The first few times I used it seemed to prove that some of the cars needed fluid changes around the 2 years and some didn't. But, in the longer run, it's only gone to prove that the fluid tends to need to be change around the 3 year period in most of the cars. The tool works by inserting it's probe into the reservoir - or a sample you've drawn off into a wee container (supplied with the kit) and initiating the test. It takes the fluid to boiling point and gives a readout on it's display. I've tried repeating the test two or three times to see how accurate it is and results are very repeatable so I believe the tool is accurate. If you are testing for repeatability then you need to take separate samples (3 in this case) before you start as the test, by it's very nature - it's a boiling point test - vapourises moisture from the sample so if you repeat the test on the same sample it will have less moisture so gives a different reading! I find also that for maximum accuracy you need to gently "swish" the end of the probe around in the fluid before pressing the button to make sure there are not any air bubbles around the enclosed heating element. Just a few years ago I spotted a special offer on a pencil type brake fluid tester. This is the other type available which tests the resistivity of the fluid by passing a small electric current through it. It's not actually a true boiling point tester. I love "gadgets" and couldn't resist it (pun intended) It's a much simpler device to use, rather like a somewhat thick fountain pen powered by a triple A battery and much cheaper to buy. In fact it has a clip, like a fountain pen, so you can keep it in your top overall pocket. Personally I wouldn't do that though as brake fluid on your overalls, considering the effect it has on paintwork, would not be a good idea - I don't wear an overall anyway, usually just a Tee shirt, so no pocket! It has two metal probes on the end and to use it you submerge the probes in the fluid and press it's button. There's a light display on the side which indicates pass/fail etc. It works, after a fashion, but is nothing like as accurate as the Suretest and much less repeatable when tested back to back. I'd use it as a very rough guide only.
So my conclusion? yes the boiling point tester is very accurate - as I expected it would be - but all it's gone to prove is that, for most vehicles, changing fluid at two years, or maybe three if your a skinflint, is a very good regime to get into. What the testers don't take into consideration is the contamination of the fluid by anything other than water, like seal particles and dirt which gets in through the reservoir perhaps. I think doing regular changes is the way to go. - and don't forget to do your clutch fluid too (if yours has a hydraulic clutch and it doesn't share the brake fluid reservoir with the brakes.) In hindsight I'd have spent my money more wisely buying my Vibroshock socket set which enables the slackening of seized cylinder nipples and forgetting about brake fluid testers and just doing a fluid change every three years, which is what I'm doing anyway, and spending the £140 on something else. Different if you're working commercially on vehicles who's history you don't know though. Then I would use the boiling point tool on ever service I did.
Edit. As most of us will know, The most common brake fluids, Dot 3, Dot 4 and 4plus are all highly hygroscopic - Take up water and moisture from any source including atmospheric air. The braking system is almost a sealed system except for the reservoir which will have a sealed cap but with an air bleed, most commonly in the cap itself, to allow for brake fluid being transferred into the system as friction materials wear. A small amount of moisture will enter via this breather, that's unavoidable, but resist the temptation to remove the reservoir cap unless unavoidable (for instance to top up or when working on the system) If you periodically remove it, for instance simply to check the fluid level, you are greatly increasing the opportunity for the fluid to absorb moisture from the air you're allowing to enter. These days the reservoirs are semi transparent anyway and if you're having difficulty seeing levels just hold a torch or light source of some type behind the reservoir.
I just thought, some reading this may not understand how dangerous and insidious moisture in brake fluid can be. Yes it's going to increase corrosion in the system so you're likely to need to change slave cylinders/callipers maybe even a master cylinder before otherwise would be the case BUT, the big problem is that water is a solid, so not compressible at the sort of pressures in a braking system so behaves, hydraulically, like the liquid brake fluid. However, unfortunately, water boils at a lower temperature than brake fluid and when water boils it changes into being a gas which is very compressible and absolutely useless in a braking system. Therefore, moisture contaminated brake fluid is a "ticking timebomb" in that most of the time the brakes will work normally because the water is liquid, BUT, under duress, so a couple of emergency stops from high speed, maybe on the motorway, or, as I found in my old Anglia going down the hill into Lynmouth, the brakes get so hot that the heat is taken into the fluid in the wheel cylinders/calipers and heats it above the boiling point of water. It turns to steam and the brake pedal, which has felt absolutely normal up to that point suddenly becomes spongy and very soon after goes to the floor as the steam allows itself to be compressed. I was very lucky and managed to pull into a run off to let things cool down. Also it had drums all round which aren't good at shedding heat - Discs are much better but can still suffer the problem in extremes. So, don't remove reservoir caps unless really necessary and renew fluid every 2 to 3 years - It's frightening how many don't do this and they're only getting away with it because the situations where the system gets hot enough are few and far between.
