why does bleeding your brakes make them better?

[sipman]

i get that bleeding your brakes makes them safer, and by refreshing the fluid you remove moisture that can boil and cause corrosion.
but why do they feel and perform better with fresh fluid? I'm assuming my cars were properly bled the prior times, so I'm not removing air, just replacing the fluid. but for two of my cars, I couldn't engage abs in the dry until I put in fresh fluid.

why is that? I thought fluids weren't compressible. even if the entire line was filled with water, shouldn't it perform the same as brake fluid at low temps?

 

[KGMtech]

It's the butt dyno effect. You change your oil and you get 10 more HP, you wax the body, you get more MPG.

Otherwise we all need to be re-calibrated.

 

[eljefino]

No seal is perfect, maybe a little air works in over time.

 

[spasm3]

Some moisture + heat= steam and air.

 

[AZjeff]

Some reading for you: http://www.aa1car.com/library/bfluid.htm

The quick answer is lower boiling point due to moisture.

When I bought the 07 GMC 2500HD with 70K 2 years ago I did a bleed/flush almost immediately based on the rep of these trucks. The pedal pressure and stopping power difference was obvious. Was so impressed I bled the brakes on the 06 Jeep Grand Cherokee (85K) and the 86 Samurai (120K). Both showed improved stopping and pedal feel, the Samurai felt like I put high performance pads & rotors on.

This bleeder set from Harbor Freight works quite well, the only thing I've ever bought from them. http://www.harborfreight.com/brake-fluid-bleeder-92924.html

Nothing to do with your butt dyno.

 

[andyd]

Gravity wise, the bleeders are the bottom of the system. Hydraulically, they are the end of the line. All the water ends up in the caliper against the bleeder. The bore gets pitted and rusty. So there must be air in there to. Air compresses, X 4 there's the squashy pedal

 

[Merkava_4]

Originally Posted By: sipman

but why do they feel and perform better with fresh fluid?


Fresh fluid is less compressible than contaminated fluid.

Why can't everybody answer the OP's question in one sentence like I did?

 

[AZjeff]

Where's the fun in that?

 

[joegreen]

Originally Posted By: Merkava_4
Originally Posted By: sipman

but why do they feel and perform better with fresh fluid?


Fresh fluid is less compressible than contaminated fluid.

Why can't everybody answer the OP's question in one sentence like I did?

Fresh fluid is less compressible than contaminated fluid.

 

[Doog]

Originally Posted By: Merkava_4
Originally Posted By: sipman

but why do they feel and perform better with fresh fluid?


Fresh fluid is less compressible than contaminated fluid.

Why can't everybody answer the OP's question in one sentence like I did?

FFILCTCF........

 

[zzyzzx]

The reason you change brake fluid is because it absorbs moisture than rusts out your brake system from inside, I think.

 

[SnowDrifter]

Hah funny. I end up convincing myself that it's worse because I'm paying way too close attention after doing it. Stuff like worrying about my pedal travel, firmness, etc. Usually end up having a friend drive my car before/after since I over-analyze it

 

[HTSS_TR]

I didn't feel much difference in braking feel after bleeding every 3-4 years. I did it for maintenance reason, to keep my brake system works as it should.

 

[sipman]

Originally Posted By: Merkava_4
Originally Posted By: sipman

but why do they feel and perform better with fresh fluid?


Fresh fluid is less compressible than contaminated fluid.

Why can't everybody answer the OP's question in one sentence like I did?

OK, so I was under the impression that fluid isn't compressible. But after reading this, I looked it up. Sure enough, water will compress up to 2% at the very bottom of the ocean. Is the suggestion that a water logged brake fluid system approaches pressures similar to what is at the bottom of the ocean? Or is the idea that the water/brake fluid cocktail is more compressible that either alone, and that accounts for the difference in feel and performance? I mean, I guess if there is no answer then I can chalk this all up to my impression of a better feel and performance, but it seems like so many people suggest fresh fluid is less compressible.

I suppose a 2% change in length over the course of the brake lines is enough to be felt at the pedal?

 

[1 FMF]

machinery lubrication i think had an article on hydraulic fluid properties and what to look for when choosing a fluid,
one of those properties was called "compressibility" or they had some other word meaning that.
I don't remember the article other than that point and remember as hyd fluid degrades by various means it becomes compressible to where it can be measured and felt. i think even new hydraulic fluids some might be less compressible than others... to a point where it can be measured and advertised whether it's significant i don't know it probably depends on the application and the higher the system pressures the more significant it is.

brake fluid pressure i think runs up around 1200 psi, or 81 atmospheres, equivalent to ~2600 feet deep in sea water. i would not compare salt water to hydraulic fluid in regards to compressibility, not much point. all fluids do compress just very very little. the problem with brake fluid is any water contamination breaks down the corrosion inhibitors and additives and then the fluid becomes more compressible as it degrades. any water located at the calipers where it gets hot turns to vapor/steam which is a gas and is very compressible.