How do bubbles enter brake fluid?

[logician]

Hi,

Question: how do bubbles enter the brake system? Obviously brake fluid can boil when heated, resulting in temporary brake fluid vapor. But when the system cools, the vapor should condense back into fluid, leaving no bubbles. So what is the explanation for bubbles forming in the brake system?

Does brake fluid go "off" and somehow change to gas state? Does brake fluid absorb air which is then later released into the brake lines, perhaps as a result of heating/boiling? Does the brake system allow air to enter the system when hot, and if so where? It seems like the only entry point for air is in the resevoir, but it seems like the resevior should be designed to prevent this. It doesn't seem like brake lines would be porous enough to allow air to enter through them unless they were also leaking fluid. Does air enter through the pistons of the master brake system or calipers? Maybe all of the above, but which contribute the most? What is the composition of bubbles in the brake system, brake fluid in gas state, air, water vapor? Does anyone know the answer?

-Juan

[turbo dave]

I'm no expert at this, but what I understand is that brake fluid (at least DOT 1-5) is hydroscopic, absorbing water whenever it isn't completely sealed. When the brakes get hot enough, the water vaporizes, giving the bubbles. Why the water vapor doesn't return to a liquid state is beyond me. Baton passed- who has the next part of the answer?

[cstreit]

The bubbles are probably a by-product of water that has absorbed into the fluid boiling off...

By all standard laws, it certainly SHOULD return to liquid form and/or return back into "suspension" in the brake fluid...

Could it be that the formulation of the brake fluid creates small enough bubbles in the "boiled water" that it can't return to it's former state because of some sort of surface tension or something like that...

Crap, I've got too much to do today and now I'll be noodling over this!

Anyone? Buehler? Buehler?

[Superman]

There is always 'free oxygen' in water (oxygen that has no hydrogen molecule to bond with). Ask any fish...that's what they breathe. And yes, brake fluid loves water very much.

[logician]

Hi,

Regarding water, or air disolved in water disolved in brake fluid, as source of bubbles in brake lines, I think that this is probably not the source. I was reading last night that DOT 5 brake fluid, i.e. Super Blue, does not absorb water. Yet, brake systems filled with DOT 5 fluid at the track do still get bubbles that require bleeding. If that were not the case, then we wouldn't need to bleed brakes so much at the track!

-Juan

[turbo dave]

DOT 5 does absorb water, DOT 6 doesn't. But, DOT 6 is compressible, losing the benefit. When you buy Super Blue, there are plenty of warnings about keeping the container sealed.

[RarlyL8]

There are many things that can contribute to air in brake lines.
- It is difficult to bleed all the air out of a system to start with.
- There are many places for water and air to enter the system over time.
- Boiling the fluid degasses it, the gas will not go back into the fluid easily.
I'm sure there are more, this is off the top of my head.

[logician]

Hi,

Regarding Super Blue and DOT-5 rating, I was indeed mistaken in stating that Super Blue was DOT-5 and therefore did not absorb water. Turns out Super Blue is glycol based and so it does absorb water and is spec'ed relative to DOT-4 rating.

DOT-5 rating is intended for silicon based fluids and silicon based fluids do not absorb water (according to Fred Puhn's brake book). But this does not apply to Super Blue.

In any event, I still question whether gas in brake lines for racing purposes is primarily the result of outgassing of disolved air, or water vapor that somehow doesn't return to fluid state. If indeed there are "many places for water and air to enter the system over time", I would like to understand what those are.

-Juan

[911pcars]

Moisture in the brake system can easily boil under braking conditions and thus cause air bubbles. However, under track or other adverse conditions, it's the heat of braking that can cause even HD brake fluid to boil and create air in the system. Heavy duty brake fluid raises the threshold where the fluid boils due to their higher dry and wet boiling points.

Hydraulic brakes depend on a solid "column" of fluid to move the caliper pistons when the brake pedal is depressed. Air is compressible, thus when there is air in the system it is the air that takes up limited brake pedal travel before the fluid column can move far enough to make the pads contact the brake rotor.

Here's a chart that summarizes many of the heavy duty brake fluids.
www.seinesystems.com/BrakeFluids.htm

In addition to the various brands of heavy duty fluid, there are various strategies to keep brake fluid from boiling. They include (in no particular order of effectiveness or cost):

Brake ducts (provides air flow directly to brakes)
Water mist cooling (used with ducts - provides cooler air to brakes)
Larger/thicker brake rotors (larger heat sink)
Larger brake calipers (larger heat sink, increased fluid capacity)
Stainless steel/titanium caliper pistons (insulates brake fluid from brake heat)
Stainless steel caliper liners (insulates brake fluid from brake heat)
Titanium brake pad shields (insulates brake fluid from brake heat)
Brake fluid recirculating system (replaces hot caliper fluid with cooler fluid in system)

Hope this helps,
Sherwood Lee
http://members.rennlist.org/911pcars