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Degassing brake fluid

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Degassing brake fluid

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flag Explodo
Posted: Mar 12, 2021 at 15:19 Quote
I happen to have a vacuum chamber. It's rated down to -29 in/Hg, but it gets close to -30. I've thrown some shimano mineral oil in there and you get a hell of a lot of gas out of it. Think it'll be beneficial or am I just playing with my tools?
flag R-M-R
Posted: Mar 12, 2021 at 16:47 Quote
Magura recommends pushing and pulling when bleeding brakes. The latter is mostly to expand any bubbles that are clinging to surfaces, hopefully dislodging them. Unsure whether that draws enough of a vacuum to pull out any dissolved gasses.

The system is normally under positive pressure only, so maybe dissolved gasses aren't a problem.
flag Kopsis
O+
Posted: Mar 13, 2021 at 6:32 Quote
Dissolved gasses aren't much of a concern since they'll stay dissolved under pretty much all operating conditions. Dissolved liquid water is a different story since braking temperatures can cause it to phase change to steam and outgas with a corresponding change in compressiblity.
flag jubs17
Posted: Sep 4, 2023 at 9:15 Quote
Explodo wrote:
I happen to have a vacuum chamber. It's rated down to -29 in/Hg, but it gets close to -30. I've thrown some shimano mineral oil in there and you get a hell of a lot of gas out of it. Think it'll be beneficial or am I just playing with my tools?

Did you ever try degassing brake fluid with the vacuum? I'm frustrated with the time it takes to properly degass with a syringe.
flag Explodo
Posted: Sep 4, 2023 at 13:44 Quote
I use the vacuum every time now. The results seemed to be worth it as far as not needing to rebleed.
flag scar4me
Posted: Sep 18, 2023 at 11:06 Quote
I always pull a vacuum and hold it on my mineral fluid syringes before I connect up for a bleed.
Normally hold it at a vacuum, twist and tap for a while till I'm happy it's all risen out up the top.
A fair amount will just be micro bubbles on the syringe internal surface, but it is always worth it.
The less dissolved gas the better!
I find this is more visible with mineral.oil than dot (fresh bottles).
flag jubs17
Posted: Sep 18, 2023 at 12:41 Quote
I struggle finding syringes that don't have crummy seals. I've resorted to putting less fluid in them for extra vacuum, and holding the syringe plunger side up so air doesn't pass through the fluid when it slips past the plunger seal.
flag adnauseam
Posted: Sep 18, 2023 at 13:28 Quote
I believe only SRAM recommends degassing DOT fluid brakes at the lever, however briefly. They used to recommend performing this step before starting the bleed.

Shimano makes no mention of it being necessary.

When I follow manufacturer instructions and perform a correct bleed the brakes work great. Why over complicate the process?
flag SPSuspension
Posted: Sep 18, 2023 at 13:58 Quote
I'm not sure how hot brake fluid can get on a maintained MTB (surely some super fast pros can boil it). But I do know that shock oil can hit 300 deg F pretty easily on our shock dyno. We don't degas suspension oil and its not common in the industry.

When I used to repair dental xrays- the tube head oil had to be degassed as the heat from the xray anode would boil thetgas causing pitting on the anode glass. But we were talking very high temps instantly.
flag Explodo
Posted: Sep 18, 2023 at 14:23 Quote
SPSuspension wrote:
I'm not sure how hot brake fluid can get on a maintained MTB (surely some super fast pros can boil it). But I do know that shock oil can hit 300 deg F pretty easily on our shock dyno. We don't degas suspension oil and its not common in the industry.

When I used to repair dental xrays- the tube head oil had to be degassed as the heat from the xray anode would boil thetgas causing pitting on the anode glass. But we were talking very high temps instantly.

Not so much worried about it boiling as coming out of suspension and forming compressible bubbles in the system. Bubbles in hydraulic works aren't good.
flag naptime
Posted: Sep 18, 2023 at 21:50 Quote
a little trick I do, even though I have NO Idea if it works or not.

I grab my beard shaver, switch it on an use the vibration to loosen trapped air. Start at the caliper then work my way up the hose to the lever (all my brakes have large lever resiviours )
flag adnauseam
Posted: Sep 19, 2023 at 10:09 Quote
naptime wrote:
a little trick I do, even though I have NO Idea if it works or not.

I grab my beard shaver, switch it on an use the vibration to loosen trapped air. Start at the caliper then work my way up the hose to the lever (all my brakes have large lever resiviours )

That's a really neat idea. I usually tap and shake the hose as I go but this does some of the work for you!

Tipping the bike up and down or dropping the caliper way down off the bike also helps with this.
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