Brake fluid

[KJM]

Just curious, but I have read that dot4 brake fluid has a higher boiling point and thus helps prevent the fluid turning to steam in the lines and then compressing causing lose of braking. Is this right? Which leads to my question...how can a given volume of any liquid evaporate and then be compressed to a smaller volume then the original liquid? If it can't then either you shouldn't loose braking pressure when boiling or the pressure the brake fluid is under should prevent it from boiling in the first place? Just idle curiousity.....

 

[dingbat]

Just curious, but I have read that dot4 brake fluid has a higher boiling point and thus helps prevent the fluid turning to steam in the lines and then compressing causing lose of braking. Is this right? Which leads to my question...how can a given volume of any liquid evaporate and then be compressed to a smaller volume then the original liquid? If it can't then either you shouldn't loose braking pressure when boiling or the pressure the brake fluid is under should prevent it from boiling in the first place? Just idle curiousity.....

Make no sense a t all
Hydraulic systems are sealed and air free.
How does fluid evaporate from a sealed system?

"Let's remember that the Department of Transportation specify the minimum wet boiling point of DOT brake fluids after absorbing only 3.7% water content (roughly 2 years service). In the graph above we can see that the boiling points of the various Shell DOT fluids decline much further over longer periods of time. When brake fluid reaches 8% water content the boiling point of Shell DOT 3 brake fluid has been reduced almost to that of water - 100°C"

 

[Scott Danforth]

yes, DOT 4 has a higher boiling point. However unless you build the brake system with all high-end, high-temp components, its a waste of time and money. DOT 3 boils at the same temperature your EPDM piston seals melt. 250 F (Mac Tilton of Tilton Engineering spent millions in the early 90s to discover this - something a friend and I discovered during lab testing in 1990).

Brake fluid is hygroscopic. meaning it will absorb moisture from the air. as indicated above, adding moisture will reduce the boiling point

if brake fluid boils, it turns to steam in the caliper or wheel cylinder. the gaseous brake fluid will back-flush the reservoir filling the line with vapor until it cools

However, when the brake system gets hot enough to get the brake fluid over 250, you have other issues, either a run-away down a mountain issue, or most likely a hanging brake.... properly maintained trailer brakes should never get the caliper or wheel cylinder that hot

no-one is doing a road course pulling a trailer. Not even figure 8 trailer racing do you need anything above DOT 3

 

[KJM]

I was more thinking about cars then trailers. I had read that a lot of newer cars call for dot4. I just don't see how a pressurized system like when brakes are applied, can boil and then cause lose of braking.

 

[KJM]

Make no sense a t all
Hydraulic systems are sealed and air free.
How does fluid evaporate from a sealed system?

"Let's remember that the Department of Transportation specify the minimum wet boiling point of DOT brake fluids after absorbing only 3.7% water content (roughly 2 years service). In the graph above we can see that the boiling points of the various Shell DOT fluids decline much further over longer periods of time. When brake fluid reaches 8% water content the boiling point of Shell DOT 3 brake fluid has been reduced almost to that of water - 100°C"

"
Make no sense a t all
Hydraulic systems are sealed and air free.
How does fluid evaporate from a sealed system?"
That was my thinking!

 

[Scott Danforth]

when the brake caliper or wheel cylinder temperature goes above the brake fluid boiling point for a period of time, the fluid goes from liquid to gas. the gas is compressible. resulting in loss of braking.

normally happens in race cars, especially on large ovals or road courses.

only happens on production cars if there is a hanging caliper constantly driving heat into the brakes.

Happens a lot on heavy delivery vehicles in chicago traffic. the term "stand on the brakes" comes from grabbing the wheel and standing on the pedal as some schmuck in a BMW just cut in front of you and you are trying to stop a brick

 

[dingbat]

Make no sense a t all
Hydraulic systems are sealed and air free.
How does fluid evaporate from a sealed system?"
That was my thinking!

Well...dug into this a bit more.
While its a sealed system down stream of the brake piston, the return circuit vents back to the reservoir which is open to the atmosphere.

Problem being, brake fluid is hygroscropic. It absorbs moisture water sitting in the brake reservoir. Once the contamination level of the fluid gets to 3.7% by volume, problems arise since water, or steam for that matter, compress. Braking efficiency would be reduced by the volume of compressible fluids in the system.

 

[KJM]

Well...dug into this a bit more.
While its a sealed system down stream of the brake piston, the return circuit vents back to the reservoir which is open to the atmosphere.

Problem being, brake fluid is hygroscropic. It absorbs moisture water sitting in the brake reservoir. Once the contamination level of the fluid gets to 3.7% by volume, problems arise since water, or steam for that matter, compress. Braking efficiency would be reduced by the volume of compressible fluids in the system.

I hear you, but when your foot is on the brake, it has to be closed thru the whole system or it wouldn't be able to pressurize. I can see the return circuit coming into play after you release the pedal.

 

[flashback]

The way I understand it is brake fluid in liquid form is not compressible but when vaporized it is compressible hence the spongy pedal.

 

[KJM]

The way I understand it is brake fluid in liquid form is not compressible but when vaporized it is compressible hence the spongy pedal.

Yes I can understand that, but how does it vaporize in a closed system when it has nowhere to expand to?

 

[flashback]

The volume hasn't changed just the physical properties of a portion of fluid. Really not much different than having a slug of air in the system..

 

[Scott Danforth]

Yes I can understand that, but how does it vaporize in a closed system when it has nowhere to expand to?

Because you don't keep your foot on the brake all the time. So 99% of the time, it's an open system

If you do keep your foot on the brakes, your mechanic and brake sales person must love you

 

[KJM]

Because you don't keep your foot on the brake all the time. So 99% of the time, it's an open system

If you do keep your foot on the brakes, your mechanic and brake sales person must love you

Ok, I see what you are saying. It gets hat enough to boil while my foot is on the brake but actually vaporizes the minute I release the brake.

 

[dolluper]

Yes boiling point is 212 so brake fluids have to have higher boiling points otherwise boiling happens and vapour created... Cools down as water.

 

[Bob_VT]

Interesting on my BMW motorcycle (and MANY others including cars) a brake fluid change is recommended every so many years. There is the "old method" where it is drained and replaced (which will raise hell with ABS) and computer support is needed to reduce ABS faults. Then there is the newer method which analyzes the fluid and determines if it needs to be changed.

I do know when the fluid breaks down it can cause internal corrosion on the brake parts.

 

[dolluper]

Gravity bleed is the best for any system... Most problems with Abs is air trapped between master and abs module esp after a master replacement...l always bench bleed the master and have a helper pump pedal three times hold and crack master lines...trapped air if any will release with this method
Then gravity bleed with farthest from master first.

 

[Scott Danforth]

I always hot-wire the ABS pump. (look up which pins for your vehicle)

 

[jimmbo]

DOT 4 does have higher Boiling point than DOT 3. One of my cars was speced for DOT 4, but then in the Owners Manual says DOT 3 is usable to top up. You should Never have to 'Top Up' the Brake Fluid, except after a Bleeding.
One way I replace some of the Fluid on a Disc System, is to open the Caliper Bleed when I push the Piston(s) back in, when replacing the Pads, and refill the Reservoir.