is it ok to flush a brake system that is currently filled with non-synthetic brake fluid with synthetic brake fluid?

So long as you do a thorough flush leaving only synthetic in the brake system (including the ABS pump) when you are finished.

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I think that all brake fluid is synthetic (non-petroleum), whether it is labeled as such or not. Can anyone confirm this?

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1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles

All conventional brake fluid is based on glycol ether's produced from petrochemicals.
There have been some silicone based fluids produced. I cant comment on their compatibility with conventional fluid. Best you contact the "synthetic" fluid manufacturer and ask.
Any DOT 3 or 4 fluid is meant to be compatible with other DOT 3 or 4 fluids.
Make sure if you are using a fluid that is not OEM type that it is compatible with the components of the system.

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I do not recommend using any silicone brake fluid in a benz. The synthetic stuff is fine as long as it has the right number of dots.

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how do you know it's non synthetic fluid in your car now? if the car's been stopping ok and you use dot 4 or 5.1 NOT DOT 5 fluid the only way to use dot 5 is to totally flush the lines completely devoid of any dot3 or 4 fluid. acetone flush followed by nitrogen flush is needed. as long as it's dot 4 or 5.1 it's ok.

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As far as I remember 3 & 4 are polyethelene glycol based and are hygroscopic (they absorb water) Dot 5 is silicone based and does not absorb water. There are issues in that any water trapped in your brake system will collect in nooks and crannys and cause spot corrosion. The two types will not mix together. If you have a brand new brake system top to bottom you can put in Dot 5 with no issues but it is difficult to completely purge the old stuff out to change over. The difference between Dot 3 and 4 is primarily the higher boiling point. Dot 5 has a higher boiling point still , in addition to a different composition. Do you need a higher boiling point in your old Mercedes, how hard are you braking? Someone else can jump in here if I have mis-remembered. Cheers Dan

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Dot 5 fluid will not absorb water (good). It will, however, absorb air (not so good). With either one you sill still have to flush it out periodically. Just not as much with Dot 5. If your car is a restored trailer queen that sees limited miles Dot 5 would be a good choice. Otherwise stick with the Dot 4. Flushing out the old stuff to use Dot 5 is simply not worth the trouble. IMHO of course.

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Silicone based fluid is NOT COMPATIBLE with conventional type fluid. Pro: it does not absorb water or damage paint. Con: it is harsher on rubber parts and braking systems must be totally free of any other type of fluid before putting it in.

I have used it on a street/track auto cross car and it has such a high boiling point that there was no brake fade. It's best suited for competition use.

Also, a dealership mechanic told me, two weeks ago, that DOT 3 & 4 are technically compatible but should not be mixed. "Use one to completely flush the system of the other." He couldn't remember the exact chemical reasons why.

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