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Mixing Brake Fluid

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Webb

Webb

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I've heard rumors that mixing DOT3 fluid and the old Castrol Mineral based fluid will eat up seals and things in British cars. However, it seems many members just put DOT3 in, and I doubt most have flushed their whole system. Is this mixing safe?
 
My understanding is that Dot 3 mixes with anything.
 
Haven't tried mixing fluids, always been told to keep it the same. With summer coming it's a good time to get the system bled through so it's ready.
 
This is the supposed mineral based fluid I've been using. Perhaps you recognize it or use it? (Click for larger picture)

 
LMA is what I have been told we should be using in our LBC's
 
Ahhh, the good stuff.
 
Yeah, it's hard as heck to find. I have to drive an hour and a half to get it. Anyway, is that stuff safe to mix with DOT 3?
 
My friendly neighborhood Advance Auto stocks LMA at a decent price. But I use "synthetic" Dot 4, personally.
 
Since the 70's that I know of, DOT 3's are mixable. That was one of those DOT rules they inflicted upon the manufacturers. That's why Castrol went from the "Girling" to the "GT-LMA" (low moisture action)

I would therefore say yes, OK to mix with other DOT-3's... but, you know there are always different experiences, opinions and data.

Just don't mix with DOT-5. That is Silicone.
 
As I remember right there on the bottle it says it is ok to mix but for the old mineral oil kind which has not been available for 20 years or so.
 
Synthetic Dot 4?????

Somebody sold you ONE weird container of brake fluid.

Dot 3 and Dot 4 are both ester based fluids, just refined a different way) Dot 3 and Dot 4 only have different boiling temps, different hygroscopic resistances.

Dot 5 is synthetic. Doesn't eat paint, very slight compressability(which you are NOT going to be able to detect in a street vehicle, properly bled). Dot 5.1 is another nest of worms.

Dot 5.5 is back to a synthetic blend but reportedly the latest and greatest as it is supposedly non harmful to some of the seals and "rubbers" that are used in braking systems. But 5.5 is hard to find. You may be able to get your friendly parts store to order it for you, but good luck.
 
Valvoline makes a synthetic that "meets and exceeds DOT 3 and DOT 4 requirements". I have used it with good results for years. It is generally considered DOT 4. It is not DOT 5. DOT 5 is silicone and controversial.

Marv James
 
Yeah, "synthetic" is just marketing crap. It's just highly refined DOT 4 that has lower moisture absorption properties and a higher boiling point.

BTW... "Synthetic" Castrol is not synthetic either. It too is just highly refined conventional oil.
 
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