Can brake fluid evaporate?

[69rangerman]

I went to take a drive Sunday and when I pulled out, the pedal went to the floor then the brakes seized hard. Pulled back in, popped the master cylinder cover and the back pot was sucking air. Didn't see any fluid underneath so bled the whole system and works fine.

Truck has been sitting for a while without driving (most of a year), with occasional start ups to keep the battery charged etc.

Any ideas?

Thanks,
Tom in FL

[fordman]

my 67 does a similar thing the brake fluisd will disapate when it sits too long. i fi put more in it it works fine. but the fluid goes away after a while. i was told the master must be bad. because i have no leaks in that system. i have been through it. i am going to replace master and booster all at the same time.

[m-mman]

Yeah, eventually ANY fluid will evaporate, but in a sealed M/C evaporation isnt going to be the problem.
Possibilites:
1. Your M/C was real low when you parked it and it lost just enough to become 'dry'.
or More likely
2. Your M/C is leaking from the secondary cup on the rear piston.

Now where did this leaking fluid go?
If you have manual brakes;
It leaked on to the rubber mat (generally easy to see but if your mat is torn and greasy already, it could be missed)
It leaked onto the carpeting. (carpet soaks up fluids so it would not be easily noticeable)

If you have power brakes it leaked into the power booster.
Think about it, if a M/C leaks out the back it will 'fill up' the power booster. A M/C will not actualy fill it, but the booster can hold enough fluid that you would not notice it missing until your brakes failed.
Where can the fluid go from the booster? When you apply the brakes a vacuum exists in the booster and the fluid can be pulled into the engine and burned.
If you pull the M/C away from the booster you might see where it was leaking out the back.

So . . . think about how old your M/C is, it might be time for a rebuild/replacement.

[70_F100]

Brake fluid doesn't evaporate...

[fordman]

ok to help him and me. out with this. my brake fluid disappears after the truck sits awhile. it isnt coming in the cab. its not running down the outside of the firewall. i have no brake booster. there are no leaks at any of the brake lines or wheel cylinders. my hood is chained shut. what is happening to my brake fluid. where is it going? i really thought it was evaporating.

[m-mman]

Everything evaporates, it is just a mater of how fast and under what conditions.

MSDS No: 503.0197992-005.009I DOT 3 BRAKE FLUID 12/12 OZ
PHYSICAL AND CHEMICAL PROPERTIES

Boiling Point
(for product) 499.3 F (259.6 C) @ 760.00 mmHg
Evaporation rates generally have an inverse relationship to boiling points; i.e. the higher the boiling point, the lower the rate of evaporation.

Vapor Pressure
(for product) < .010 mmHg @ 68.00 F

Specific Vapor Density
6.000 @ AIR=1

Percent Volatiles (Including Water)
2.8 %

Evaporation Rate
< .01 (BUTYL ACETATE =1 )
http://www.ilpi.com/msds/ref/evaporationrate.html

So it does evaporate . . . . but it aint much and it aint fast.

When brake fluid is lost in stored vehicles it is generaly "leaking" somewhere.
Not a major leak. Probably nothing you can even see. Rather it is a seepage.

If brakes are used regularly (even just stepping on the pedal once a month) the seals/cups are continually pushed out against the metal bores, if they sit, the seal lips will shrink ever so slightly and allow for a slight seepage.

[69rangerman]

Thanks guys. I'll keep an eye on it...and add the mental note to the "one more thing to do" list.

[19FORD67]

Seems like I was once told that brake fluid can absorb moisture from the air.I guess this could effectively lower its boiling point thus increasing evaporation.

[Hawkrod]

Seems like I was once told that brake fluid can absorb moisture from the air.I guess this could effectively lower its boiling point thus increasing evaporation.

Nope, it does not work that way. If the water is absorbed it increases the volume and it would evaporate before the brake fluid would. The reality is if you lose brake fluid in a year then you either have a leak or the brake shoes/pads have worn enough that the cylinders need more fluid (usually something you only see on disc brakes or drums that are not self adjusting). If your fluid goes down fast enough to need fluid regularly then you better start looking for the leak. As noted above, often the fluid will leak out of the back of the MC but it does not always show. Very often it simply runs down behind the floormat and soaks the felt backing. Hawkrod

[19FORD67]

Thanks Hawk. Sorry if I'm hijacking here; so when brake fluid absorbs moisture what happens? Is there a condition that tha culminates from moiosture in the brake lines?

[70_F100]

That's generally what causes the pitting and rust inside wheel cylinders and master cylinders.