Brake Fluid Flush, Synthetic?

[ronspradley]

I am going to flush the brake fluid in my 93 Gulfstream MH. Should I use synthetic? It is not supposed to absorb water and that is the problem with regular fluid. This MH does not get used as much as it should and sits for much of the year. Do not think it matters, but it has discs front and drums rear, air over hydraulic.

Thanks for the input.

ronspradley

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[R2Home]

Years ago when I was working on vehicles, we were informed NOT to mix the two different types of brake fluid. If you are going to rebuild the whole system, drain all of the fluids and put new fluid in the system you might be OK.
No matter the fluid that is used, the hydraulic bakes system should be flushed to keep it working properly. It might be possible that the tow fluid can be mixed but lets hear from those that are currently in the business.

[scenic route]

DOT 3 & 4 are designed to be hygroscopic(suspends the moisture it comes in contact with). With variations in temperature, moisture will always be a problem for brake systems. Todays brake fluids suspend this moisture and also contains corrosion inhibitors that help protect system components. That is why the experts recommend flushing brake fluid every couple of years. Mixing is not recommended.

[ronspradley]

I do not want to mix the regular brake fluid with synthetic, I want to flush out the old, and replace with synthetic. Is that recommended?

ronspradley

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[scenic route]

Ron, I understand that Prestone makes a DOT 4 synthetic but not sure what is required for a proper flush. Maybe a call to Prestone would be in order for the straight story.

Manufacturer: Prestone Products Corp. Div., Honeywell International, Inc. Address: 39 Old Ridgebury Road City: Danbury State: CT Zip Code: 06811 Telephone Number: 203-830-7800 Fax Number: 203-830-7904 Toll Free Number: 800-890-2075

[Nick-B]

Let's get some terminology straight.
First, ALL brake fluid is synthetic. Don't let container labeling confuse the issue.
I think what is meant here is the difference between silicone DOT5 and conventional DOT3 & 4 fluid. Definitely don't mix those. DOT3 and 4 are compatible and the main difference is 4 has a higher boiling point, but only when it is fresh and not contaminated with moisture. Once it absorbs some moisture it has a lower boiling point than 3.
Don't use DOT5. It is for racing applications only.
If you want to use DOT4 just do a normal fluid flush. A trace of DOT3 won't hurt anything.

[Hikerdogs]

You should stick with either DOT3 or DOT4 fluid. DOT5.0 is a silicone based fluid that is not compatible with the rubber components in your system. There's also DOT5.1 which is used on newer vehicles that isn't silicone based, but I'm not sure whether it's compatible with the older rubber components.

Contrary to popular belief the brake system isn't totally sealed. The cover on the reservoir isn't air tight. Moisture will enter the system through the reservoir and dissipate through the fluid. The DOT3 and 4 fluids are designed to hold moisture in suspension. If they didn't the water would accumulate at the low points in the system and freeze in cold weather rendering the brakes useless. In warmer weather the water will remain at the low points and cause rust and/or corrosion.

While fluid containing moisture will boil at lower temperatures it will continue to work until it gets saturated. Saturation is considered to be when the volume of water in the fluid reaches the 3% range. This usually doesn't happen for several years. That's why the manufacturers recommend changing the fluid and flushing the system every 2 years.

[ronspradley]

Thanks for the responses. After reading here and elsewhere, I have decided to just flush and re-fill with DOT4 fluid. Recommendations I was reading suggested only switching to synthetic silicone based fluid when doing a complete re-build, replacing components and seals. Not doing that at this time.

ronspradley

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