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!!

Well, like we used to say in the Army: "If you got 'em, flush 'em!".  ;-) That is, you are always gonna wanna flush that system from time to time.  There's other stuff going on in there besides insidious water vapor.  Best clue is as mentioned: the color. It will get dark. Through all of this, I can't see using DOT5 for anything.  DOT 4 seems like a good idea.  I finally got a simple suction pump brake bleeder thingy (Advance Auto, cheap) that allows to do it by yourself.  Seems to work fine.  Pull out the dark stuff and any bubbles, until the light color comes through, no bubbles.

Hi all,   I finally got around to removing/replacing the leaking master cylinder.  Easier than I had expected with the car up on a lift.  Thank you again for your help!

After removal and inspection of the old MC It was evident the seals had been damaged.  Small rubber chucks were visible in the fluid dripping out of the MC on to the paper towel while on the bench

Chatting with the old school mechanic that was on duty at The Shop ( where I rented the lift stall to work on my car), he came up with a feasible theory as to what may have happened, based on other similar failures he had seen over the years,

his thought;

- the MC was Brazilian made, Vargas brand.

- southern hemisphere rubber is more susceptible to cold temperatures, and depending on where the rubber came from it will harden/shrink a bit  ( more than northern hemisphere rubber) when temperatures dip close to/below freezing.

- since I tend to drive in the cold months, often times on nice sunny 35F-40F days, it is likely I have regularly taken the car out when the MC rubber seals are hard ( harder than they should be) and have slowly damaged the hardened cold rubber seals. Around the edges and contact points to the metal MC body/parts ( explaining the “ fine rubber dust and chunks that came out of the MC fluid)… eventually getting big leaks when the temp drops and the damaged seals harden/shrink.

- I went with a euro sourced MC and brake switches.  Will see how this one fares to cold temps.  Driving home today it was 33F, and it started to snow as I pulled into my garage.   Driving down to The Shop yesturday it was 27F… but super dry, sunny and clear).  Car suns great when it’s cold outside, and the heater boxes do a decent job of keeping me warmish.  

thanks again,

cheers,