RE: Time for a brake flush. What fluid are people using? - 2

BMW - Bring My Wallet..

I let the dealership carry out this bi-annual and a two year service if required, less the oils, which they are ok to do.

Oil/filter/bevel changes are over-serviced at home, sort of a mongrel mix but works for me, and I ignore the annoying 'service spanner'

Perhaps I should invest in one of those 'widget things' for a home reset but as I still rely on a paper sheet of phone numbers I am best to leave well alone!

I did change the 'lifetime' clutch fluid after 10 years on the R1200ST, easy to do and cleaning the reservoir of debris at the same time but noticed no change in performance..

Zitat von Paul911 im Beitrag #8

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Zitat von TriangleRider im Beitrag #7

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Zitat von Paul911 im Beitrag #1

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, cycle the ABS unit with the GS911 and then flush again..

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What does this accomplish?

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If you have replaced brake components, cycling the ABS unit moves the fluid and helps get air out of that. If it's just a regular brake flush, it's probably unnecessary but it's fairly easy to do so I might as well.

I'll add one other step I'm going to do. I'm going to put the new brake fluid under a vacuum. My father has done this (MIT educated engineer who worked in the defense industry his whole career) and he's found when he's done it with new fluid, dissolved air comes out.

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Not sure if the current abs units are as easy as older kit but applying the brakes overnight (cable ties/weight on the levers) and then any residual air was likely to rise through a system, espicially of you had the engine running, a little vibration helped as the pressure released much like a bottle of cola.

The only way to get brake pressure on my neighbours old R100RS was to remove the rear caliper, tie it to the seat before bleeding system.
Used a block between pads and then the restricted brake line air lock loop around the swingarm no longer caused the air lock.
It took us two bottles of brake fluid before we saw this problem although the front fluids were far easier to replace.

Zitat von Mr. 36654 im Beitrag #17

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The potential impact of the variation in gaseous cavitation would be what?

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Zitat von Paul911 im Beitrag #19

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Zitat von Mr. 36654 im Beitrag #17

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The potential impact of the variation in gaseous cavitation would be what?

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Are you referring to the step of degassing the brake fluid, by either vacuum or ultrasonic, prior to the brake flush?

If so, the dissolved air can come out at higher temps as liquids can't hold as much dissolved gas at higher temps. So, your brakes get squishy when hot. Additionally, and this is my father's theory, the fluid is less likely to oxidize over time. He was a physicist though, not a chemist.

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Zitat von wessie im Beitrag #20

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does DOT4 oxidize?

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Zitat von Mr. 36654 im Beitrag #21

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First of all, most all fluids under vacuum will release entrained gases. Similarly, increasing pressure will drive those same gases back into solution. So, unless you degas the fluid and keep it in a sealed container or system, that gas will return to the fluid. That's why they tell you to use a fresh, sealed bottle of fluid when doing a brake flush.

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Zitat von wessie im Beitrag #24

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did you miss the bit about additives to inhibit oxidation?

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Zitat von Paul911 im Beitrag #23

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Zitat von Mr. 36654 im Beitrag #21

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First of all, most all fluids under vacuum will release entrained gases. Similarly, increasing pressure will drive those same gases back into solution. So, unless you degas the fluid and keep it in a sealed container or system, that gas will return to the fluid. That's why they tell you to use a fresh, sealed bottle of fluid when doing a brake flush.

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That I agree with, which is why I would do this immediately before using the fluid to do the brake flush. Also, as I said, the process was done to fresh brake fluid (i.e. an immediately opened bottle) which then showed evidence of having dissolved gas.

So the question would seem to be how long it takes for the same amount of air to redissolve in the fluid and how quickly does that happen inside the brake system.

The engine isn't a sealed system so I'm not seeing the point on the oil.

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Zitat von Paul911 im Beitrag #22

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Zitat von wessie im Beitrag #20

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does DOT4 oxidize?

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https://www.sciencedirect.com/science/ar...03238619788175X

Yes, at least one component of DOT4 does oxidize.

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Zitat von Mr. 36654 im Beitrag #26

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Zitat von Paul911 im Beitrag #23

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Zitat von Mr. 36654 im Beitrag #21

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First of all, most all fluids under vacuum will release entrained gases. Similarly, increasing pressure will drive those same gases back into solution. So, unless you degas the fluid and keep it in a sealed container or system, that gas will return to the fluid. That's why they tell you to use a fresh, sealed bottle of fluid when doing a brake flush.

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That I agree with, which is why I would do this immediately before using the fluid to do the brake flush. Also, as I said, the process was done to fresh brake fluid (i.e. an immediately opened bottle) which then showed evidence of having dissolved gas.

So the question would seem to be how long it takes for the same amount of air to redissolve in the fluid and how quickly does that happen inside the brake system.

The engine isn't a sealed system so I'm not seeing the point on the oil.

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My point about the engine was just that...........it's not a sealed system and the oil and engine survives! On the other hand, power steering fluid operates in a closed system and gets the crap beat out of it with the control valves and vane wheel pump.

Relative to the rate of absorption, it's a matter of the exposed surface area and the amount of dissolved and free (entrained) air (larger voids). I'm guessing your 1-cycle degassing is primarily extracting entrained air. Removal of dissolved gas usually requires several pressure cycles.

If you want to pursue this molehill, you really should spend some time researching cavitation phenomenon.

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Zitat von Paul911 im Beitrag #29

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Zitat von Mr. 36654 im Beitrag #26

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Zitat von Paul911 im Beitrag #23

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Zitat von Mr. 36654 im Beitrag #21

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First of all, most all fluids under vacuum will release entrained gases. Similarly, increasing pressure will drive those same gases back into solution. So, unless you degas the fluid and keep it in a sealed container or system, that gas will return to the fluid. That's why they tell you to use a fresh, sealed bottle of fluid when doing a brake flush.

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That I agree with, which is why I would do this immediately before using the fluid to do the brake flush. Also, as I said, the process was done to fresh brake fluid (i.e. an immediately opened bottle) which then showed evidence of having dissolved gas.

So the question would seem to be how long it takes for the same amount of air to redissolve in the fluid and how quickly does that happen inside the brake system.

The engine isn't a sealed system so I'm not seeing the point on the oil.

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My point about the engine was just that...........it's not a sealed system and the oil and engine survives! On the other hand, power steering fluid operates in a closed system and gets the crap beat out of it with the control valves and vane wheel pump.

Relative to the rate of absorption, it's a matter of the exposed surface area and the amount of dissolved and free (entrained) air (larger voids). I'm guessing your 1-cycle degassing is primarily extracting entrained air. Removal of dissolved gas usually requires several pressure cycles.

If you want to pursue this molehill, you really should spend some time researching cavitation phenomenon.

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So what do you make of this observation? When put under vacuum (using a typical 25 micron vacuum pump for AC evacuation), I see a lot of frothing. Once that subsides, if I shake the bottle I see more frothing. Eventually with continued vacuum that subsides.

I understand cavitation at a rudimentary level but I'm not sure how it applies to liquid in a vessel under vacuum.

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