Brake Master Cylinder Query

@Lfepardo- Scott's 2000 Tacoma is converted from 2w to 4wd with an older Toyota straight front axle and leaf springs (it is the definition of a 4wd riding like a tank!) and I'm pretty sure the guys who built it used dot3.  To be honest, I don't think it matters which brake fluid is in it. The 2012 SI Civic came with dot4 (I think- a short look online was inconclusive) so putting dot3 in it, while not quite right, isn't a major faux pas (it hasn't messed anything up).  When I can talk him into bleeding the brakes again (it's probably about due for it's 2 year fluid change any way- we did front and rear brakes in it a couple? years ago) we'll use dot4.

From what I've read, a lot of new cars (starting in 2006) came with dot4.  I haven't seen anything about new cars with dot5 from the factory, but haven't investigated any of the higher end brands (Porsche, BMW, Mercedes Benz...).

@Stan Galat- I never read anything about dot4 being more hygroscopic than dot3 (mind you, again, I didn't delve that deeply into it), so interesting info.  I've always believed brake fluid turns dark when it absorbs water (I don't know where I got that from) so as long as the container stayed tightly sealed I've trusted what came out of the old container on the shelf.  I do like your idea of always buying the smallest amount needed...

@ALB posted:

@Stan Galat- I never read anything about dot4 being more hygroscopic than dot3

I don't think I said that DOT4 was more hygroscopic. I said RACING brake fluid was more hygroscopic.

@Stan Galat- I re-read what you wrote, and my apologies! Thank you for correcting me.

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@ALB posted:

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...Armed with this new knowledge, I'm now wondering what I'm going to do with the almost full (hey, I just went and checked!) quart of dot3 I bought earlier in the late summer/early fall...

Not so fast, Al.

I just checked and Dot3 is three hundredths of a gram lighter per cc than Dot4.

Some quick math shows you could save over an ounce of weight in the whole system by sticking to Dot3.

Whatever you do, don't drill the reservoir.

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@Sacto Mitch posted:

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Not so fast, Al.

I just checked and Dot3 is three hundredths of a gram lighter per cc than Dot4.

Some quick math shows you could save over an ounce of weight in the whole system by sticking to Dot3.

Whatever you do, don't drill the reservoir.

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Touché Mitch!!!

You guys are too funny.   I wonder how mineral oil will work on these cars?  ;-)

All new parts arrived today… so this may be a weekend project if the kids don’t want to go skiing tomorrow-Sunday, or a spring project if they do, as they ski 3-4 days a week, and I am back to coaching at the UW.  Never a dull moment!!