Can brake fluid evaporate?

I had this discussion with my boss, he says over time brake fluid will NOT evaporate. I say it will, what is your opinion??

One would expect virtually all liquids to evapourate. Of course this is neither a yes or no.

I do know that brake fluid absorbs water from the atmosphere. I assume that if conditions are right the fluid would allow the water to evaporate at certain humidies and temperatures and absorb it at others. This may give the impression that the Brake Fluid itself is evapourating even if it does not.

How is that for a round about way of saying "I don't know"?

I know that you can experience a slight loss of brake fluid from your reservoir over time without an actual leak causing it.

For the record, if you can smell something from a fluid then that indicates that some component of that fluid is outgassing. So, yes, brake fluid, hydraulic fluid, water, power steering, gasoline... all fluids will evaporate under the right conditions.

Just to clarify..When your brake fluid goes down in the resevoir that is usually due to the pads and shoes wearing and thus their static positions change and more fluid is in the calipers and cylinders and less in the resevoir.

As DRW1000 said: the normal "loss" of brake fluid is into the caliper. The caliper is simply a piston in a cylinder. This piston moves the pad out against the rotor. As the pad wears down, the piston has to push it further out against the rotor. There is (usually) no functional return spring to return the pad to it's "original" position, just the absence of pressure. This causes there to be a slight drag on your brakes at times, but the cylinder is full of fluid, so that any application of pressure (by the pedal and master cylinder) will cause braking.

The system below the master cylinder is sealed tightly, and capable of handling hundreds of psi. The master cylinder is sometimes vented to allow fluid to move, as needed, up and down in the resevoir. Being that brake fluid is not water based (typically), I can't imagine anyone would ever see noticable evaporation.

To see the effect of the change in the amount of fluid retained in the caliper, watch the level in the resevoir increase when the calipers are compressed (to the original position) in order to install new pads.