Glad it’s sorted
Most brake fluids such as DOT 3 and DOT 4 are hygroscopic. Being hygroscopic means
they will absorb moisture when exposed to air. Such moisture can be absorbed through brake lines, through seals or via the brake fluid reservoir. Such occurs even when the vehicle is not driven. That’s why manufacturers provide a time based schedule for brake fluid replacement.
The main enemy of brake fluid performance as such is moisture absorption. But not in the way some think… It’s not because water within the brake fluid suddenly turns the fluid into a compressible liquid, that’s impossible as all fluids are incompressible. The problem with water in the brake fluid is that water boils at 100C while brake fluid boils at 230C.
So with a few applications, even long light applications, lots of heat is generated within the brake system that ends up in the brake fluid within the caliper.. With the likelihood of the water within the brake fluid decreasing boiling point over time, ultimately yje water within will boil away and turn to vapour. The more braking, the higher chance of the water boiling and turning into vapour
And that’s where the problem is, when water boils it turns into water vapour which is highly compressible which results in brake fade…
As a side, DOT3 dry boiling point is 205C while its wet boiling point, that is 3.7% water within the brake fluid, is lowered to 140C. (3.7% can relate to 2 years of service)…
| Dry Boiling Point | Wet Boiling Point | Composition |
---|
DOT 3 | 205°C/401°F | 140°C/284°F | Glycol Ether |
DOT 4 | 230°C/446°F | 155°C/311°F | Glycol Ether/Borate Ester |
DOT 5 | 260°C/500°F | 180°C/356°F | Silicone |
DOT 5.1 | 260°C/500°F | 180°C/356°F | Glycol Ether/Borate Ester |
The more water within, the lower the boiling point and the easier it is to suffer brake fade…
Even with dry DOT5.1 brake fluid, which boils at 260C, drive as if you’re on a track and you’ll put lots of heat into the system and you’ll suffer brake fade.
So, if one sufferers brake fade, always check for fluid loss and if there is none then suspect moisture contamination and change fluid as the first corrective action. And if one still has problems, one can then look master cylinder (low line pressures?) or brake booster (vacuum?) or even driving style (if they think they’re Fangio) as the cause…
The real difference between DOT 3 and DOT 4 are their boiling points, which are narrowed to dry and wet boiling points. Do you know the difference?
blog.amsoil.com
Learn the difference between the 'dry' and 'wet' boiling points of brake fluid and what it means for your braking performance.
epicbleedsolutions.com