Dot 4 Brake Fluid

Looking at different brake fluids that are available I ran across this stuff. Impressive wet and dry boiling points. Anybody here use it.

Use to. Went with Dot 5.1 just because. Don’t race or tow, so probably will never notice the difference.

I don’t think I ever use my brakes hard enough to require the high temperature specs.
So I use standard DOT and flush it every two years thinking that fluid renewal contributes more important to system longevity.

Dennis 69 OTS


As above. Biannual flushing to replace water saturated fluid is much more important than the absolute boiling point difference between Dot 3 and 4. FWIW, Dot 4 absorbs water faster than Dot 3, and should be flushed annually, or every 90 days for track applications.

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When I restored my car I was able to flush, clean and rebuild all components, so I went with silicon based dot 5 and haven’t had an issue in 36 years. Brakes and clutch both.


Speaking of DOT X fluid, I found it very interesting to see in the manual of my new ‘21 Turbo Mazda 6 that it requires DOT 3 fluid!!

I think it’s more important to flush it regularly than what you use. I use Napa 3/4 dot or Valvoline 3/4, I think.

Actually, Dot 3 absorbs moisture right through the rubber hoses, so you’ll have water in the system eventually, hence the need to flush. I haven’t heard that to be necessary with silicon fluid, though flushing the system every few years is a good idea anyway. Recommended every two years or 30k miles.

Tom, do you flush the DOT 5 on a regular basis, and assuming you do, how often?

I’ve never done it! Since 1985! No issues with pedal action and purging, nor performance.

It was always an issue with my MK2 with dot3. Bleeding the system and topping off. Replacing corroded components.

Then you have had great success. I had used DOT 5 30 some years ago. Worked great, but a little spongy. After maybe 20 years without flushing, the brake MC started to get a little sticky. So I rebuilt the brakes. The DOT 5 had become very dirty and sludgy over the years. Because of the brake feel, I now use DOT 4 and flush every 2-3 years. I do not know how long this method will last, but the brakes feel better. Glad it has worked for you.

There are at least two great debates in old car circles, motor oil and brake fluid.
I suspect that in Tom’s case, that after 20 years the brake parts needed rebuilding anyways regardless of what fluid he used. I have had good experience with DOT5, but I only install it in a clean and completely rebuilt system. You also need to treat it carefully as it can foam easily causing a spongy pedal. One argument against DOT5 is that water will get into it. DOT5 is actually hydrophobic, where DOT4 is hydroscopic. It’s your car, use what you like. Sorry to open a can of worms…

Yes, and notice I did not condemn DOT5. I do not know all the factors involved. But I do not understand your statement

Before I installed the DOT 5 the brakes were rebuild. So from what I can tell, Tom Taylor’s system and my system both started at the same condition. His is still running, I felt mine needed rebuilding. Why? Possibly there are explanations:

  1. Brand of DOT 5. Maybe his was better than mine.
  2. Storage. mine had been stored in a garage, but no heating- 0 F to 110F. Maybe Tom T’s was not? I live in Pennsylvania. We do have high humidity. Do not know about Tom T’s.
  3. Usage. I do use mine every year. But limited mileage. Maybe Tom T uses his constantly. Maybe shows only? Maybe when used, I get mine extremely hot, maybe Tom T does not, or maybe the opposite.
  4. Perception. Maybe I was displeased with the brakes, the feel. Maybe another person would have been fine with them.
    This is not to make any excuses, etc. Just that I believe the “debate” will continue. As in other items, there may not be a “best” brake fluid. But a brake fluid that is best for a particular car’s situation.

Back when I did the restoration, research indicated that the system had to be rebuilt, with no old fluid stuck anywhere in components, tubes or hoses. The two fluids are not compatible, plus the new fluid will dislodge any contaminants and pedal loss or heart-stopping intermittent braking/clutch action will result.

In my case every component was removed and cleaned or replaced, and sleeved calipers were installed.

I live in the Pacific Northwest and drive the car in spring through fall. I keep it in a heated and dehumidified garage.

I don’t feel that the action of either brakes or clutch are spongy, but someone else might disagree. I’ve driven this for so long, I’d like to experience a similar car with dot 3 to see how much different it would be.

Here in Blighty fluid condition is a testable item - the fluid must be clear and clean. Dirty or cloudy fluid is a MOT failure. I change the fluid every two years regardless of mileage, and I always use 5.1. It ain’t much more expensive than 3 or 4, but it will last longer if I find myself driving swiftly down the Stelvio Pass next summer.