May 12th, 2009
How not to top up your brake fluid
On the top of my Ducati’s front brake master cylinder are the following words:
“Use only DOT 3 ÷ 5 Brake Fluid”
And as the following post will demonstrate, this does NOT mean DOT 3 or DOT 5.
First, a brake fluid primer. There are three commonly available brake fluids: DOT 3, DOT 4 and DOT 5. One would presume that the higher number means that it’s “better” vluid, but this is not necessarily so. DOT 4 does have a higher boiling point than DOT 3, and most mechanics will not hesitate to substitute DOT 4 for DOT 3. The two also mix just fine, so there’s no problem topping up a DOT 3 system with DOT 4 fluid.
DOT 5 is where things get complicated. DOT 5 is silicon based. It is not hygroscopic. It does not mix with DOT 3 or DOT 4. In fact, mixing DOT 5 in a brake system that is currently filled with DOT 3 or DOT 4 is nearly catastrophic.
You can guess where this is going.
See, the Europeans are crafty, and what they really meant was DOT 3/5, which is the European way of saying “DOT 3″. To make things worse, the service manual calls for DOT 4.
And you guessed it: I topped it up with DOT 5.
I realized my mistake almost immediately, when the purple DOT 5 fluid combined with the DOT 4 fluid already in there to make these odd creamy-colored globuoles. And if you’ve ever had that light-headed tingly feeling when you know you’ve really fucked up – well, you’ll know how I felt that afternoon.
What followed was 10 excrutiating hours of disassembling every single component in my bike’s brake system. I’m talking about caliper pistons removed from calipers, delicate rubber O-rings from the deepest corners of the brake system, and even a disassembled master cylinder that took about ninety-six fingers to hold together when I had to re-insert the final spring clamp that holds everything together.
No, fair reader, it was not a fun job at all. The odor of aerosol brake cleaner still fills my garage, and the only good thing is that I now have the cleanest front brake system in North America.
At least, until the next poor sod dumps DOT 5 fluid in his decidely non-DOT-5 brake system.