How does air and water get into brake lines anyway?

[sugarwood]

We've all read that brake fluid is hygroscopic. But, this is a closed system, otherwise, you'd have brake fluid leaks and the brakes wouldn't even work. So, how does water ever get into the system in the first place?

Also, since its closed, how does air ever get into the lines?

[ben parrish]

Water from the atmosphere...vent in the cap.

[sugarwood]

Yea, but the air is above the fluid level in the reservoir.
How would it get under the liquid and into the lines?

[Ronnie's.930]

Sugarwood, as Ben alluded to, hydraulic brake systems are not closed (none that I have ever seen anyway, on motorcycles, cars or small aircraft) - don't think they would work if there was no way for the systems to release pressure to atmosphere.

As far as air getting into the hoses and hardlines; the only time I've ever heard or experienced that happening is if there is a slight leak somewhere (such as in the master cylinder). If no leak, then the air was in the system all along (not purged properly in the first place).

[sugarwood]

If they are not closed, wouldn't there be a leak sprung when you pressed the pedal?

How would air get in there on the first place?

[MTemp]

Water vapour in the air in the reservoir is absorbed into the brake fluid, and the water molecules being heavier than the brake fluid slowly sink lower in the system. As you pump the brakes fluid is also slowly moved around and this will enhance the mixing process. Also as you drive the car the motion of the car will continuously change the air in the reservoir introducing more water vapour to be absorbed into the fluid..... and so on. Slow process, but over a couple of years enough of it goes on to potentially compromise your brakes.

I've taken a few technical liberties to keep it short, but hopefully you get the drift.

[Driver_X]

And should the brake fluid get hot enough with heavy use, the water in the fluid will actually boil and leave very small air pockets. In addition, water is compressible while hydraulic brake fluid is not.

[yelcab1]

Under your theory of the brake fluid is a closed system and therefore no air or contaminant can get in, then the fluid would never look disgustingly dark after a year. Yet, we all know that it does look dark after a year so it obviously picked up something, most likely water.

[Driver_X]

And note, while the hydraulic part of system itself is closed, the reservoir is open to the atmosphere to allow for expansion of the fluid due to heat.

[Jcslocum]

Originally Posted by Driver_X

And should the brake fluid get hot enough with heavy use, the water in the fluid will actually boil and leave very small air pockets. In addition, water is compressible while hydraulic brake fluid is not.

Sorry to disagree, but water is not compressible, at least in this application.

Air can enter thru the seals in the master cylinder or the caliper as the molecule is smaller and therefore would not show a fluid leak.

[mreid]

Air does not get into a properly working brake system. You bleed the brakes after replacing a component or doing something that allowed air to enter. Brake fluid absorbs water and as stated above, eventually absorbs enough that it can corrode and damage the system. The more aggressive you drive, the more heat/cool cycles your brakes go through and the faster this occurs. This is why you change it periodically, more often if racing.

[sugarwood]

Originally Posted by MTemp

Water vapour in the air in the reservoir is absorbed into the brake fluid, and the water molecules being heavier than the brake fluid slowly sink lower in the system. As you pump the brakes fluid is also slowly moved around and this will enhance the mixing process. Also as you drive the car the motion of the car will continuously change the air in the reservoir introducing more water vapour to be absorbed into the fluid..... and so on. Slow process, but over a couple of years enough of it goes on to potentially compromise your brakes.

I've taken a few technical liberties to keep it short, but hopefully you get the drift.

Ok, so the air in the reservoir is destined to absorb into the fluid over time. Thanks for clarifying.

Originally Posted by Driver_X

And should the brake fluid get hot enough with heavy use, the water in the fluid will actually boil and leave very small air pockets. In addition, water is compressible while hydraulic brake fluid is not.

Boiling makes sense, but few people ever do this in real life.

Originally Posted by yelcab1

Under your theory of the brake fluid is a closed system and therefore no air or contaminant can get in, then the fluid would never look disgustingly dark after a year. Yet, we all know that it does look dark after a year so it obviously picked up something, most likely water.

No, brake fluid does not get dark after one year.

Originally Posted by Jcslocum

Sorry to disagree, but water is not compressible, at least in this application.

Air can enter thru the seals in the master cylinder or the caliper as the molecule is smaller and therefore would not show a fluid leak.

If water is not compressible, why is water bad? B/c it has a lower boiling point?

Ok, so air can enter through dry seals like at the caliper? Thanks for clarifying.

Originally Posted by mreid

Air does not get into a properly working brake system. You bleed the brakes after replacing a component or doing something that allowed air to enter. Brake fluid absorbs water and as stated above, eventually absorbs enough that it can corrode and damage the system. The more aggressive you drive, the more heat/cool cycles your brakes go through and the faster this occurs. This is why you change it periodically, more often if racing.

Doing what exactly allows air to enter?
Changing pads and rotor clearly does not.

[Jcslocum]

Yes, water boils at 212 and brake fluid at 400 or so depending on what you are using. so this v=creates steam and a spongy pedal. Water also causes rust and pitting in the brake system, this is bad too, obviously.

Depending on the condition of the seals, the pumping action of the caliper seal and the master cylinder seals can allow air into the system over time.

That little rubber bellows inside the master rez cap is supposed to help keep air away from the fluid but it's a system that has many brake fluid to seal to air points in it and they can all introduce tiny amounts of air over time.

[Joe Bob]

Originally Posted by Driver_X

.............. water is compressible while hydraulic brake fluid is not.

Water is compressible? Aspirating water thru an intake will shatter rods and grenade an engine when it hits the combustion chamber if in enough quantity.

[Jerome74911S]

Maybe I should go out to the workshop and double check, but I believe that there is a vent tube running from the reservoir all the way down to the bottom of the car - in my case. Isn't that there to allow for the venting of excess fluid volume, when very hot? And when the fluid cools, air would be drawn back in to the reservoir, so there is always a supply of fresh air right at hand. I'll go out and have a look. Obviously, enthusiastic use of the brakes is required to cause this cycle to occur. Or, do I not know why there is a tube running to the bottom of the car?

[boyt911sc]

Originally Posted by Jerome74911S

Maybe I should go out to the workshop and double check, but I believe that there is a vent tube running from the reservoir all the way down to the bottom of the car - in my case. Isn't that there to allow for the venting of excess fluid volume, when very hot? And when the fluid cools, air would be drawn back in to the reservoir, so there is always a supply of fresh air right at hand. I'll go out and have a look. Obviously, enthusiastic use of the brakes is required to cause this cycle to occur. Or, do I not know why there is a tube running to the bottom of the car?

Jerome,

The brake fluid reservoirs for 911 has a vent or over flow tube expose to atmosphere. I have not seen a brake fluid reservoir for '74 to '89 without vent. But for satisfy your curiosity, take a look.

Tony

[Ronnie's.930]

Originally Posted by sugarwood

If they are not closed, wouldn't there be a leak sprung when you pressed the pedal?

How would air get in there on the first place?

The system is not closed, otherwise once you activated the brakes, they would remain locked.

I don't understand what you mean by "leak sprung", but the system is open to atmosphere at the high point (the reservoir vent) - every hydraulic brake system I have ever seen is as well. The position of the master cylinder piston, when the brakes are applied, keeps fluid from shooting back up into the reservoir and out its vent (maybe that's what you mean by "leak").

Air gets in upon initial assembly of the system, each time a component is replaced and often when fluid is changed and/or bled (like MReid mentioned).

Originally Posted by Driver_X

And note, while the hydraulic part of system itself is closed, the reservoir is open to the atmosphere to allow for expansion of the fluid due to heat.

The system is not closed to atmosphere - would not work if it was (as I noted above).