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dluyk
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adding synthentic break fluid?
01/05/12 at 01:20:42
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I'm getting ready to do my first rear break job. Pulled the hub - went easier
then I thought-whew!

I am planning on replacing the wheel cylinders and associated hardware.

I know I will need to bleed the breaks and add brake fluid - but don't know if i can use synthetic fluid to fill
the master cylinder which may have hydraulic fluid in it.

Can I add synthetic to hydraulic? As always thank you for your knowledge and help.
  
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athawk11
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Re: adding synthentic break fluid?
Reply #1 - 01/05/12 at 03:04:58
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I don't believe it's generally a good idea to mix regular brake fluid with synthetic.  Same rule that applies to real oil mixed with synthetic oil.

I drained my entire system and used fresh DOT 5.  If your system has not had fresh fluid for some time,  it's worth draining the whole thing and adding fresh regular or synthetic brake fluid.  My fluid was almost black with contamination.

Warning:  Brake fluid will remove paint.  Really well.  (I should have used it to strip my Willys.)

Tim    
  
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Re: adding synthentic break fluid?
Reply #2 - 01/05/12 at 03:50:07
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Don't mix them, just use Dot 5.
  
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aboyandhisdog
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Re: adding synthentic break fluid?
Reply #3 - 01/06/12 at 01:39:36
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Interesting that you guys are recommending Dot 5.   My knowledgeable guy at NAPA says Dot 3 is the stuff.  Dot 4 is Dot 3 with a higher working temperature apparently.  Don't know much about Dot 5 but that is what I use in a newer car.  And isn't it all considered "synthetic" anymore?  Help...I'm so confused!!!
  
Tom
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Re: adding synthentic break fluid?
Reply #4 - 01/06/12 at 01:52:33
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Oops., Sorry.   DOT3 is what I used.  It is synthetic, which is fine, but older non synthetic mixed with synthetic, I believe, is a no-no.

Tim
  
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Re: adding synthentic break fluid?
Reply #5 - 01/06/12 at 02:10:01
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Just did a search and DOT3, 4 and 5.1 are what is normally used and are moisture absorbent. DOT5 is synthetic. I just agreed with Tim thinking that is what I use too but I just looked and I use DOT3. The different numbers are boiling temps.
  
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Re: adding synthentic break fluid?
Reply #6 - 01/07/12 at 15:51:25
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I think the word "synthetic" might be a bit misleading. DOT3 and DOT4 are glycol-based fluids, and DOT5 is silicon-based. DOT3/4 absorbs water, DOT5 does not which makes it good for brake systems that sit out in the weather year round or are stored where it may be cool/cold and damp. The difference between DOT3 and DOT4 is the boiling point. DOT4 has borate esters added to increase the boiling point.

There are pluses and minuses to DOT5. On the plus side, of course, is it does not absorb moisture and it's boiling point is higher. On the minus side is it's much more difficult to get the bubbles out so it's very important to be very careful when adding the fluid to the MC that you not create bubbles when you pour. And for heavens sake don't let the MC run dry while you're bleeding the brakes! Another minus is that the hotter the brakes get, and subsequently the brake fluid, the spongier silicone fluid brakes will feel. Another minus is that if ant moisture does get into the system it stays as pure water and does not mix with the silicone therefore it can lower the overall boiling point and can create rust pockets within the system.

The best time to convert to DOT5 is when you are replacing the entire braking system. You can not mix DOT5 with any other version. If you have an existing system and want to convert there is a way but it's a lot of work and involves flushing the entire system, usually with alcohol.

Generally speaking DOT4 is more than good enough for standard flat fender applications.

Larry
  
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Re: adding synthentic break fluid?
Reply #7 - 01/07/12 at 17:18:01
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I totally agree with you on that Larry. I won't be switching to DOT5 in my Jeep.
  
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dluyk
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Re: adding synthentic break fluid?
Reply #8 - 01/07/12 at 22:05:21
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thank you for all the input. I'm going with dot 3.
Dave
  
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Howie
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Re: adding synthentic break fluid?
Reply #9 - 01/17/12 at 13:59:44
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Thanks Rocketeer, this is one reason I love this site.  One can learn so much and all of you are so willing to help as you can.  No progress on Frankenjeep, it's waiting patiently for the refurb.....y'all know how it is.

Thanks again, Howie
  
Howie
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