Differences between DOT3/4 fluids

[Todd H 26112]

The thread below re silicon reminded me of something I didn't discover in the archives. When it comes to standard (non silicon) fluids most folks seem interested in dry or wet boiling points from brand to brand. Has anyone ever discovered if there are differences in the rate at which various brands of std fluids absorb water? I believe the term is hydroscopic. E.g. given the same environment can brand X differ from brand Y w/ respect to how much it absorbs over time? And if so is the information available or does anyone have any recommendations for others?

This might be more of interest to folks who don't run silicon and have a classic car (or newer car) that is unlikely to see track runs or something.

thanks,

[Brian Monticello]

Re: Differences between DOT3/4 fluids

Todd,
I'll add little value here by saying yes. This assumption is based on the fact that Castrol (I think) sells an LMA DOT4 brake fluid. LMA stands for Low Moisture Absorbtion. Could be marketing hype.

Hopefully Duke or someone else will provide the chemistry behind it.

Brian

[Terry M.]

Re: Differences between DOT3/4 fluids

Todd,
There are many differences. The dry boiling points are: DOT 3 = 401 deg F (205 deg C), DOT 4 = 446 deg F (230 deg C), and DOT 5 = 500 deg F (260 deg C).

There are many other differences, and as is pointed out some brands claim a lower moisture absorption rate than others. There is also a DOT 5.1 poly glycol fluid that has been introduced recently, but I was unable to find those standards quickly. I have been told that the standards for DOT 5.1 rival those for DOT 5. Perhaps others will do the internet search to verify that.

The amount of work required for this research is extensive, and I would welcome the results as a story for The Corvette Restorer. I have papers from SAE on research done on Silicone fluids that could add much to such a story, for anyone who has an interest in pursuing the area. I hope this link to the Federal Motor Vehicle Safety Standards comes through. It would be a start, but barely scratches the surface.




Federal Motor Vehicle Safety Standards

[Todd H 26112]

Re: Differences between DOT3/4 fluids

Yep - that's exactly what i'm after - quantifying the absorption rates.

The DOT ratings are only minimums. I've read test results that show some DOT3s have higher boiling points than some DOT4s - for whatever reason the manufacturers have not bothered to certify at the higher DOT4 levels. The infamous Ford DOT3 HD fluid is or was a good example of that.

But I've never had success finding truly quantifiable differences among fluids w/ respect to absorption rates.

Years past I followed the silicon debate to teh point that i became utterly ambivalent on the subject and stuck w/ DOT3/4s.

Also in years past my searches on brake fluids always led to the boiling points but rarely is serious discussion given on absorption rates. I figured some folks here that are into vintage cars might be in the know...

The link by the way is a keeper - am digging thru it presently. Thanks.

[Mike M.]

Re: Differences between DOT3/4 fluids

Terry: its hygroscopic, not hydroscopic as a previous post guessed at. regards, mike

[John H.]

Re: Differences between DOT3/4 fluids

It's also siliCONE, not siliCON; a Las Vegas topless dancer lying face-down on the beach is the only time the two ever meet

[Al Corelli]

Thank you for the clarification, John...

I would have done it, but I post here too infrequently to be taken seriously.

AC

[Duke W.]

Re: Differences between DOT3/4 fluids

Brake fluids are chemically similar to ethylene glycol antifreeze, and like
antifreeze, they have corrosion inhibitor additives. I am under the impression that DOT 4 will provide better corrosion protection than DOT 3s, but I'm not sure if it's science or marketing hype. Gathering information on brake fluids is one of those things that I do, but I don't have enough to write a comprehensive article. My SWC has silicon fluid, but I run DOT 4 on my other cars as I have never had a reason to take the systems completely apart, and I flush the fluid every two years, usually with Castrol LMA DOT 4.

When I upgraded the brakes on my Cosworth Vega to Monza vented rotors and their companion larger piston calipers circa 1981 I took the rear wheel cylinders apart and found that they already had some corrosion!!! I replaced them and got religious about flushing the fluid every two years. Other than going through about six sets of front brake pads on the CV (about 4000 miles or race track hot laps) I have never done anything else to the brakes other than fluid flushes, so it's 22 years and counting. The fluid usually doesn't even darken unless I take it to the track, and the cast iron M/C reservoir is squeaky clean.

Duke

[Todd H 26112]

Re: Differences between DOT3/4 fluids

I personally run either Valvoline Syn or Castrol LMA or Ford DOT3 but I've never been able to make up my mind about 'absorption rates' and name brand vs whatever. That's just one aspect of comparing fluids I've never personally seen quantified.

If I may ask, since your switch to silicon what is the duty cycle of your SWC? If one is completely overhauling their brake system is it worth considering? I recall this was a hot topic in the past but wonder if things have changed much or new aspects are known...?

I had a car that I dabbled w/ at autoxing and such and bled teh hydraulics periodically. Hydraulics in clutch failed twice regardless and at least one brake MC failed. But just a few weeks ago I helped a family member do a brake job in my shop on a near 20 yr old Pontiac that started to have squeeling brakes w/ ~125K. These were original factory pads - yeah the fluid looked dark and nasty but I guess it worked. Go figure.

thanks,

[Duke W.]

Re: Differences between DOT3/4 fluids

The duty cycle on the SWC brakes is zero, since it's still under restoration, but the brakes are together. I think switching to silicon is a good idea if you are going to take the brake system completely apart and start with everything clean and dry. Periodic flushes should still be done, but maybe every five to ten years instead of every two, depending on your climate and driving conditions. If the car is not driven in rain or frequently exposed to temps below the dew point, then you can go a long time between flushes.

Most old cars have brake fluid that looks like week old coffee and you can scoop sludge out of the bottom of the M/C with a spoon; some have never had a hydraulic leak, but, trust me, these brake systems are living on borrowed time and a failure is iminent.

Duke