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Tech Question

Gary Boak, Chilliwack, BC, Canada, 1969 Dart Swinger 340

I notice in the April 04 issue you say that DOT 5 brakefluid can be used without flushing the system. Everything I have heard andread states that Dot 5 does not mix with anything except DOT 5. Whenswitching to DOT 5 are you not supposed to use new rubber?


Another old wive’s (or Mopar Muscle) tale. First, realize that DOT specsdon’t specify the composition of the fluid, the rating relates only to theboiling point. So, therefore, it would be possible to have DOT 3 fluidthat’s silicone, and DOT 5 that’s not (and this latter combination doesexist.) These specs are covered in detail in Federal Motor Vehicle SafetyStandard section number 571.116, which is part of the Code of FederalRegulations, #49, Chapter V, see for yourself at:

Code of FederalRegulations, #49, Chapter V

Here’s an excerpt from the law on boiling points:

Equilibrium reflux boiling point (ERBP). When brake fluid is testedaccording to S6.1, the ERBP shall not be less than the following value forthe grade indicated:

(a) DOT 3: 205 ̊C. (401 ̊F.).

(b) DOT 4: 230 ̊C. (446 ̊F.).

(c) DOT 5: 260 ̊C. (500 ̊F.).

A recent change to the law specifies that non-silicone fluid meeting DOT 5 temperature specs. be labeled DOT 5.1. All DOT 5 fluid must be purple in color, all other grades must be clear to amber.

The baziilion pages of the specs relate mostly to boiling points, viscosity,and the fluid’s effects on rubber (swelling), as well as testing procedures.But there’s also much written about compatibility. What has probably causedthe confusion is that DOT 5 fluids are tested differently than other types,but these tests apply equally to silicone and non-silicone fluids.Subchapter S6.5.4 addresses miscibilty, specifying that the fluid beingtested must mix with a standard type fluid, and this miscibilty test DOESapply to DOT 5 fluids, and can’t gel, swll cups, etc. when mixed. So there!

The Society of Automotive Engineers (SAE) also has detailed specs: J1703 andJ1705.

It’s true that much has be written saying that silicone based fluids cannotor should no be mixed with other types, this is that old myth-perpetuatingdeal. Nowhere can I find a scientific study or analysis to back this storyup – do a Google search yourself if you don’t trust me.. There’s only beentwo studies on this that I’m aware of: Dow-Corning, the leader in siliconebrake fluids (possibly the only true USA manufacturer), did a detailed studyalmost 30 years ago, wherein a system was haphazardly swapped from DOT 3 tosilicone, with the intent of leaving a significant portion of the fluidUNchanged. The system passed all DOT tests easily. The second test is lessscientific, but, to me, more significant: Yours truly has been using DOT 5silicone in every car I’ve owned or serviced since approx. 1978. I havenever flushed a system! I have, in some cases, bled out most of the oldstuff, in other cases, I’ve just topped up a DOT 3-filled system withsilicone. Cars serviced by me in this fashion have stopped will from speedsover 160 MPH. I’ve never had a stuck caliper (or wheel cylinder) pistionsince making this change universal in my fleet – even in cars stored forlong periods. I’ve also never damaged pain from a brake fluid spill. Come tomy garage – you will find 4 or 5 bottles of DOT 5 silicone, and zero of DOT3 or 4.

On your Dart, do what I do (and say): bleed out as much of the old garbageas you can, until you see purple at each bleeder screw. And don’t worryabout it!

End of story.



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