Nagging brake fluid curiosities

[andyd]

My brake bleeds/flushes happen after line or caliper R+R. I buy DOT3-4 BF by the qt. The stuff picks up moisture, so I use it all. Gravity bleeding proves out my work. A system with air in it wont bleed. Rehabbing calipers and wheel cylinders(waste of time) shows where the bad brake fluid accumulates. Most wheel cylinders cant be rebuilt because the pits are too deep. Calipers are built differntly and the bore pitting doesnt bother anything. It takes water and air to make rust. Sucking the highest and lightest 1/2 cup out of the MC and replacing it with fresh BF is ineffectual. The bad stuff is the sludge accumulating at the bottom of the system. That is what you need to get rid of. I also use an airlock made out of some clear tubing and a hot sauce bottle. Submerge the hose end in a little BF hitch t'other to the bleeder. Open bleeder and pump the pedal by hand. Watch for bubbles. I mostly just pump the container near full. BF is about 8$ a qt and it doesnt keep. Either one of these cheap and easy methods have given me the best pedal.

 

[Yup]

I used to just crack the bleeders every 2-3 years, grab a beer, and keep the MC topped off. Great cheap way to get fresh fluid in there. Then I saw a Mighty Vac on sale for $29 and bought it. Amazing how fast you can do it with the Mighty Vac. It takes longer to get the tires off and bleeders open than to pull the fluid through.

 

[circuitsmith]

Originally Posted By: dailydriver
Originally Posted By: circuitsmith
Originally Posted By: Nate1979
So why does the fluid in the reservoir become darker over time?


Contaminated fluid at the calipers can diffuse all the way to the reservoir.
It can take years, but it happens.


But it will not work the other way around (i.e.; clean, new fluid put into the reservoir diffusing to the calipers even after years and years)?

What will happen is the fresh fluid will diffuse to the calipers and dilute the contaminants.
That's a little better than nothing, but there's still the chunky crud that's held down by gravity.