Service History

[alohart]

Under $20 for a "brake fluid tester" that dips into the fluid in the reservoir. (Hygrometer may not be the correct term for it, as that appears to relate to gasses not fluids.)

That sounds like a brake fluid tester that measures the electrical conductivity of the fluid. Unfortunately, this type of tester might not be very accurate because different brands and types of brake fluid have different electrical conductivities when freshly poured out of their cans due their different additives and to the different compositions of DOT 3, 4, 5, and 5.1. Some of of these electrical conductivity testers claim to account for different brake fluid types with settings for each type while most don't. The depth that the electrodes might also affect their readings. The voltage of the battery that powers these testers might affect their accuracies with older, more used batteries having lower voltages.

Reading some comments on YouTube videos that evaluate these testers indicates that these testers are really hit and miss and probably shouldn't be depended on.

 

[tordon]

ALL brake fluid absorbs moisture

Not all of them, the DOT5 (5.0) is silicon based and not hygroscopic and does not mix with water. That might seem like the solution to the problem with water in the brake fluid, at least it did to me, so i used it in my Volvo 145 -71, which does not have ABS. It says on the bottle that DOT5 is not suitable for ABS, eventhough I know that a couple of my friends used it in ABS cars, and the ABS functioned normally, they said.
Using DOT5 in my Volvo did not end well, after a number of years (15?) without brake fluid replacement the car needed a couple of new calipers and a new master brake cylinder, plus a complete overhaul of the other two calipers, all due to rust.
Water comes into the brake system by condensation of water vapour in the air that enters the reservior through the breather hole, like Alohart says, and, I suspect, at the calipers when wet pistons go back into the cylinders after braking in rainy weather. This happens regardless if the brake fluid is hygroscopic or not, and the water can, besides causing rust, also freeze at low temperatures and block the passage of brake fluid, with unpleasant consequences, if the water isn't absorbed by the brake fluid.
So the brake fluid being hygroscopic and mixing well with water is a good thing, because then we don't get the problem with ice in the brake system and we can easily get rid of the water that inevitably enters, by just replacing the fluid.
I have come to the same conclusion as Alohart and Colin123, every third year I buy ten litres (2,2 imperial Gallons, 2.6 US Gallons) of Bosch ENV6 and have a brake fluid replacement orgy on my five cars (the i3 among them) and my two motorcycles.

 

[mojo]

Using DOT5 in my Volvo did not end well, after a number of years (15?) without brake fluid replacement the car needed a couple of new calipers and a new master brake cylinder, plus a complete overhaul of the other two calipers, all due to rust.

That's an odd one - I've used silicone brake fluid on 3 cars, and never had any issues. I was told that before using it, you need to strip down all of the system components - you must not have any trace of the original brake fluid or you will get the worst of both worlds!

I rebuilt the calipers myself, using approved silicone grease and silicone brake fluid, and that (along with the rubber dust caps) should ensure that no water enters the system anywhere in the road wheel area.

I had reason to dismantle and rebuild those calipers after around 22 years (admitted not many miles though) and there was absolutely no sign of rust anywhere inside them.