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Old 09-15-2006, 10:46 PM
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Brake fluid "evaporation"

**background info...
Earlier this year my grandpa figured he'd back out his 67 Mustang that he never drives and hasn't seen the road since purchase in the early 80s. (convertible, 289, 3sp, ugly factory hubcaps) Well, he got the old thing to fire up, and was backing it out, and there was absolutely no brakes on it... so after he managed to get it to stop he checked the brake fluid... the cylinder was absolutely empty. There was no signs of leakage ANYWHERE.
This is coming from a retired GM R&D guy (he worked with lifters at the Wyoming Mich. GM plant back in the day, ceramic lifters and neat stuff like that) as well as an after-hours mechanic on the side, later becoming ASE certified until 1991 as the last certificate on the garage wall states.

This made me wonder...
**to the point now...
On my underfunded project car, the same thing is true- no sign of brake fluid anywhere... no leaks or anything! And a few years back my mom drove it for a winter since the engine in her daily beater died and needed rebuilt. (thanks mom... gotta love rust ) Well it hadn't been ran in a few years prior to that, and there was no brake fluid in that either... fill and bleed the brake system my dad tells me, and there was no problem since.

Let it sit for another ten years... 1996 to 2006.

Same thing- no sign of leakage, just no brake fluid once again.

**my question-
So where does this brake fluid "dissapear to?" It shouldn't evaporate as it's petroleum based and rather heavy at that... and would the Jav likely be ok with some fresh fluid and an afternoon of brake bleeding?

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Old 09-16-2006, 02:41 AM
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Be careful...

I have had cars do this after sitting awhile. Turns out the fluid in the master DOES absorb moisture from condensation, it rusts up the master bore and the fluid then leaks into the car thru the firewall, and runs down under the carpet padding. You may have just gotten lucky in that putting in fresh fluid and bleeding gets you a pedal, but I'd bet it's still leaking inside the car, where it's hard to see. To be safe, replace the master, they are cheap compared to a crash if the seals fail completely...

The fluid does'nt evaporate at all, you can set a dish of it out for years and it WILL absorb moisture, to the point the dish gets FULLER if you marked a fluid line level on it.

If the master is leaking the fluid is gonna run out the end seal where the pushrod is. And don't think the aluminum cylinders are exempt, they get a whitish corrosion that just wipes the seals out if you just put fresh fluid in...
Motorcycles are really prone for this. I have had luck with older bikes to just disassemble and carefully scrub the corrosion out and lightly hone the cylinder with a Scotchbrite, and reassemble with the (cleaned-up) original seals, where a rebuild kit was impossible to locate.

With iron cylinders the rust starts on the bottom of the bore, because that is where the water is concentrated. My feeling is that if the end seal is leaking, the piston seals are also working against this rust pitting, and will fail in short order.


Last edited by DrChop; 09-16-2006 at 02:50 AM. Reason: adding something...
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