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Brake fluid: can synthetic and regular mix?


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10 replies to this topic

#1 RegalBM

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Posted 21 May 2008 - 11:49 PM

I went to Autozone this afternoon trying to buy some regular DOT-3 brake fluid, but surprisingly saw a lot of synthetic fluid, like Prestone and Valvoline, and the regular was rare. The guy at AZ said synthetic is compatible with regular and they can mix. I remember I once read an article on the web saying, once the synthetic fluid is used, it may not be possible to change back to regular any more. Any comments?

Thanks.
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#2 Scimmia

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Posted 22 May 2008 - 01:18 AM

There is no such thing as non-synthetic brake fluid. Some companies use "Synthetic" for marketing, some don't, but it doesn't matter either way.
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#3 attist178

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Posted 22 May 2008 - 03:44 AM

I'm too tired to explain "synthetic" brake fluids...it's complicated and it has big words...lol.

But no, don't mix them. And don't listen to parts store guys...the majority of them have absolutely no idea what they're talking about.

A good rule of thumb: Don't mix stuff with different names.
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#4 Scimmia

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Posted 22 May 2008 - 01:30 PM

I'm too tired to explain "synthetic" brake fluids...it's complicated and it has big words...lol.

But no, don't mix them.


I'd be VERY interested in hearing the explaination, or atleast getting a reference. This is contrary to everything that I've found about brake fluid. Polyethylene Glycol is by it's very nature synthetic, so there is no brake fluid out there that's not synthetic.

Unless you were thinking of DOT5 (silicon) fluid vs DOT3/4, which isn't what we're talking about here.
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#5 RegalBM

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Posted 22 May 2008 - 03:28 PM

OK, can I do this as a bottomline? Pump the old brake fluid out from the master cylinder reservoir and then put the synthetic fluid in. Since can't get all the old fluid completely out from the brake system, there will still be some mix but it seems to me it should be much better. Would this be fine?
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#6 oakleafresin

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Posted 22 May 2008 - 03:47 PM

For what its worth, I flushed my brake lines out last year with "Synthetic" Prestone fluid from Autozone and I havent had a single problem stopping. I used a Mitty Vac. I took as much of the old crap out of the master cylinder as I could and refilled it with the Prestone. Then I bled each caliper till clear(ish) fluid came out.

A couple weeks ago I rebled them using this process and the fluid looked opaque and dark brown (rather than black/green) almost like rust. I dont know wtf the deal was with that but the car is still stopping great lol. Im sure dark brown is a normal color though. Would probably turn green eventually.
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#7 Scimmia

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Posted 22 May 2008 - 09:55 PM

OK, can I do this as a bottomline? Pump the old brake fluid out from the master cylinder reservoir and then put the synthetic fluid in. Since can't get all the old fluid completely out from the brake system, there will still be some mix but it seems to me it should be much better. Would this be fine?


Bottom line is that all DOT3/4/5.1 fluids are compatible, and all synthetic. As long as you don't get DOT5, you're fine. I'm guessing that attist178 was confusing synthetic with silicon.
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#8 Dusty

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Posted 23 May 2008 - 05:05 PM

+1 on mixing DOT 3/4/5.1 Just say away from DOT 5 silicone
If you have not flushed the fluid recently I would just get a big bottle of Dot 3 or 4 synthetic and change out the fluid. Easy to do with a helper to push the brake pedal.
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#9 RegalBM

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Posted 03 June 2008 - 01:33 AM

I hope this is the ultimate answer...I got a chance to talk with a good friend of mine today, who holds a chemistry PhD and is a technical team leader in the R&D department of Valvoline. He said no problem to mix the synthetic with the regular ones (I am not going to elaborate his explanation here).

Just wanted to post this fyi and hope it helps.
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#10 Guest_LSChris_*

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Posted 03 June 2008 - 06:16 AM

+1 on mixing DOT 3/4/5.1 Just say away from DOT 5 silicone

^^X2
DOT 5 silicone fluid is not compatible with conventional brake fluids and they will seperate over time if mixed. DOT 5 silicone brake fluid can't be used in systems with ABS because the action of the ABS unit will induce air into the system.
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#11 RegalBM

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Posted 03 June 2008 - 04:11 PM

+1 on mixing DOT 3/4/5.1 Just say away from DOT 5 silicone

^^X2
DOT 5 silicone fluid is not compatible with conventional brake fluids and they will seperate over time if mixed. DOT 5 silicone brake fluid can't be used in systems with ABS because the action of the ABS unit will induce air into the system.


Yes, my friend meant DOT-3/4/5.1 and no mix with DOT-5.
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