In the August Issue | Brake Fluid: 9 Questions and Answers

David S.
By David S. Wallens
Jul 1, 2020 | brakes, Hygroscopic, Brake Fluid

What does brake fluid do? 

In the simplest of terms, brake fluid transfers the pressure generated in the master cylinder by the driver’s leg all the way to the brake hardware found at each wheel of the car. Ideally, this fluid should not freeze, boil or excessively compress, and it should remain compatible with the materials found in the system. It’s a tall order.

What does hygroscopic mean?

The glycol ether stocks used to produce DOT 3, DOT 4, DOT 4+ and DOT 5.1 brake fluids, by their very nature, absorb water from the atmosphere. Introducing less than 5% water to brake fluid can sometimes chop its boiling point by nearly half. Related note: Once a container of brake fluid is unsealed, use it ASAP and dispose of any remainder.

Why is brake fluid hygroscopic?

Our atmosphere contains moisture, and no matter what, that water will find its way into a brake system, likely passing through the pores of the rubber hoses and parts found within. If this moisture isn’t absorbed by the brake fluid, it can collect as liquid water at the system’s lowest points, like the calipers and wheel cylinders—and with that comes rust. These puddles can easily freeze or boil, too, causing further problems with the brake system’s components.


Subscribe to Grassroots Motorsports magazine today to get the August issue and read this full story. Find out what the different brake fluid standards mean, what's so special about DOT 5 brake fluid, what separates one racing brake fluid from another, and more.


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View comments on the GRM forums
parker Reader
6/30/20 3:22 p.m.

Has the August issue mailed?


ChrisTropea Associate Editor
7/1/20 10:24 a.m.

In reply to parker :

The August issue is shipping now. If you subscribe by today, July 1, 2020, you should receive this issue. 

Ranger50 UltimaDork
7/1/20 10:59 a.m.

I hope I get mine.

spandak HalfDork
7/1/20 3:06 p.m.

I read the article and it was good! 
I have one question that I cannot find the answer to though:

Is there any downside to using brake fluid higher than the called for spec? For example, my wife's car calls for DOT 3. Any reason I should use DOT 3 over the bottle of DOT 5.1 I have on the shelf? 

Ranger50 UltimaDork
7/1/20 3:22 p.m.

In reply to spandak :

I never mix different fluid levels. If it's dot3, I keep putting in dot3. Now I've switched to dot4 in my Avalanche and Suburban so I keep them with their like manufacturer fluid as the differences are great between manufacturers.

Matt B (Forum Supporter)
Matt B (Forum Supporter) UltraDork
7/6/20 11:14 a.m.

In reply to Ranger50 :

As long as you "flush" out all the old fluid through bleeding I would think it shouldn't matter if you put in a higher spec, but that might not be contrary to what you're saying.  As far as mixing, I also assume I'd simply never get the benefit of the better fluid if it wasn't pretty uniform in there. 

I never thought about the differences between manufacturers.  I've never experienced any pain points on the issue, but I can't imagine it being a big deal on a street car (could be wrong I guess).  That said, my track cars have all had uniform spec & manufacturer fluids, but admittedly that's partly a byproduct of the replacement cycle between events.

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