Why does changing brake oil correct spongy brakes??

[Andeh]

Both mineral oil and DOT brakes will eventually get spongy after about a year, even if they're not ridden much. And if you ride a lot, you'll need to bleed them more often. I'd chalk it up to no seals being perfect.

I've had mineral oil brakes pull to the bar/wandering bite point (cough Shimano) and get spongy/have poor lever feel (Magura) and I've had DOT brakes get spongy or require moving lever/bite point out every ride (SRAM, Hayes). All of them got better with a fresh bleed.

[XXX-er]

1st time eva bleeding the guides I shook up the bottle of finishline 5.1 for a few minutes figuring it would release all the air bubbles from suspension and there was a LOT of air in the 5.1

then I watched a bunch of U-toobe's but decided to follow the DH mechanic kid with hair like lyle lovett who said don't follow those guys with logo's on their shirts cuz you don't need to do that

basicly just pushing/pulling fluid thru at the caliper at the bottom till there were no bubbles going into either the bottom syringe or the top cup and its been a good bleed

IMO/IME on cars and bikes we have always bled brakes to get all the air out of the system, sure you could probably reuse the fluid but the idea is to get contaminants ( air/ rust/ dirt/ water) out of your brake system

[El Chupacabra]

1st time eva bleeding the guides I shook up the bottle of finishline 5.1 for a few minutes figuring it would release all the air bubbles from suspension and there was a LOT of air in the 5.1

Don't ever do that. You introduced a bunch of tiny bubbles into the brake fluid.

[XXX-er]

Yeah I've heard that ^^ one but how can you be sure you aren't shaking the bubbles out of suspension ?

AND BTW its too late but the lever has been hard as a rock for a couple months so the bleed must be good, how long before that lever goes flacid again ?

[MagnificentUnicorn]

Don't ever do that. You introduced a bunch of tiny bubbles into the brake fluid.

They haven’t heard about aeration in Canada


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[J. Barron DeJong]

If you’re really worried about that (which you shouldn’t be), find some way to pull a vacuum instead. Would be simple enough to rig something up if one was so inclined (which they shouldn’t be).

[joetron]

If you’re really worried about that (which you shouldn’t be), find some way to pull a vacuum instead. Would be simple enough to rig something up if one was so inclined (which they shouldn’t be).

Pull a vacuum in the syringe before the bleed and purge the gas. Easy peasy.


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[J. Barron DeJong]

Pull a vacuum in the syringe before the bleed and purge the gas. Easy peasy.


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Ok. But I don’t use a syringe, so what now

(I hope that if I did use a syringe, I would have realized that was the obvious solution here.)

[XXX-er]

The real reason for buying a bleed kit is to get all the fittings that will screw into your brakes and you want to pay a little more for the metal fittings but all the bleed kits i have seen come with at least 1 syringe, the pro sram kit comes with 2, you can buy a large syringe from the pharmacy for 2$ or they might even give you one,

don't forget to tell the pharmacist you are not a drug addict you are just fixing your skis/ bike

[J. Barron DeJong]

The real reason for buying a bleed kit is to get all the fittings that will screw into your brakes and you want to pay a little more for the metal fittings but all the bleed kits i have seen come with at least 1 syringe, the pro sram kit comes with 2, you can buy a large syringe from the pharmacy for 2$ or they might even give you one,

don't forget to tell the pharmacist you are not a drug addict you are just fixing your skis/ bike

My kit came with a syringe, I just don’t use it. I do the remove-the-bleed-screw-completely-gravity-bleed method.

[XXX-er]

There are just slightly less brake-bleeding vids than there are total # of you tubes on the internet

everyone has a slightly different take on the process and some have golf shirts with nicer logo's while i just wanted " the method " so I expected to watch a vid and giturdone, but what i found is a bunch of U-toobes that are all just slightly different than the last one so I had to figure out wtf is really going on here as opposed to just following a recipe,

so I watched them all and I picked buddy the DH mechanic with the large hair cuz i figured he ain't gona be wrenching for a DH racer if he can't make the brakes work and so buddy is sayin you don't need to do that stuff in them other vids AND it worked i no longer have flacid brakes


Right so you said earlier you don't use a syringe well you can always use that syringe to shoot heroin

[J. Barron DeJong]

There are just slightly less brake-bleeding vids than there are total # of you tubes on the internet

everyone has a slightly different take on the process and some have golf shirts with nicer logo's while i just wanted " the method " so I expected to watch a vid and giturdone, but what i found is a bunch of U-toobes that are all just slightly different than the last one so I had to figure out wtf is really going on here as opposed to just following a recipe,

so I watched them all and I picked buddy the DH mechanic with the large hair cuz i figured he ain't gona be wrenching for a DH racer if he can't make the brakes work and so buddy is sayin you don't need to do that stuff in them other vids AND it worked i no longer have flacid brakes


Right so you said earlier you don't use a syringe well you can always use that syringe to shoot heroin

Similar story here. Also went with the DH mechanic:



His rider seems good enough that he could discern between a good and bad bleed too, eh?

Been working for me so I’ve just stuck with it.

I’ll keep the heroin tip in mind for when I have to travel to Baltimore for work.

[XXX-er]

I spose mineral oil isnt so nasty as dot 5.1 but its a nasty looking job ^^ I think I would have worn gloves, I think that auzy mechanic has a hangover and now he has to have his tucker with a side of shimano brake fluid

I'm rocking Sram so maybe the syringe method is better for that, in any case do what works for ya, in my case that was a syringe for the Dot5.1 ,

shaken ( not stirred )

A few yers ago I hit that same very nice pharmacist up for a FREE medium syringe to inject 2 part into a Tech toe pullout/ top sheet delam so they do have a good use besides shooting heroin

[jamal]

Couple things

1. Don't shake up your brake fluid- that is definitely adding air into it.

2. 3.7% is just where "wet" boiling point is measured. The fluid can have more water in it than that.

3. that's also a large benefit to dot fluid over mineral oil- with the mineral oil the water is always just in there as water doing it's thing. Another note is that the dot boiling point are a minimum requirement for that particular type (ie 3, 4, 5.1) along with other things like viscosity. There are plenty of fluids that have much higher boiling points.

4. To answer the original question, water isn't the only thing getting into your brake fluid over time- tiny little air bubbles, like the ones we try to remove by pulling vacuum on the syring, get in there too, and would be the main cause.

[J. Barron DeJong]

Couple things

1. Don't shake up your brake fluid- that is definitely adding air into it.

2. 3.7% is just where "wet" boiling point is measured. The fluid can have more water in it than that.

3. that's also a large benefit to dot fluid over mineral oil- with the mineral oil the water is always just in there as water doing it's thing. Another note is that the dot boiling point are a minimum requirement for that particular type (ie 3, 4, 5.1) along with other things like viscosity. There are plenty of fluids that have much higher boiling points.

4. To answer the original question, water isn't the only thing getting into your brake fluid over time- tiny little air bubbles, like the ones we try to remove by pulling vacuum on the syring, get in there too, and would be the main cause.

Yes to 3.7 being the measurement point, but my understanding is the saturation point is close to that, under 4% (happy to be corrected here though).

Most racing fluids are DOT 4 and have boiling points way above the minimum requirements for DOT 5.1. I assume they’re still DOT 4 because they don’t meet some other requirements (probably viscosity since DOT 5.1 is very low viscosity to work with high frequency ABS systems.)

[Eluder]

2. 3.7% is just where "wet" boiling point is measured. The fluid can have more water in it than that.

This was my understanding as well, but after looking, they consider 3.7% saturation... so I conceded. Still, after reading more, it's not referring to full saturation in the scientific way we are talking. It is my understanding that brake fluid can indeed absorb more water beyond this point. It's just that anything beyond 3.7% is considered unsafe as the boiling point exponentially drops to an unsafe point very rapidly. Different fluids absorb at different rates, obviously.

[toast2266]

I'd be interested to see a measurement of how much water is actually in your average brake line. I'm assuming (probably incorrectly) that water ingress into a brake system is probably similar from brake to brake, regardless of whether it's DOT or mineral oil. So if you took a sample after, say, a year of riding, how much water would be in the system?

My guess is that it'd be far less than 3.7%. Most cars have an open braking system, so the fluid is presumably a lot more susceptible to absorbing moisture. But most car manufacturers only recommend changing brake fluid every few years (and plenty of people go way longer than that without any massive negative consequences). If car brake fluid was hitting the 3.7% saturation point, I would think manufacturers would recommend a more frequent flush. So based on that, it seems pretty unlikely that bike brakes with their closed system are getting anywhere near 3.7% water content.

[J. Barron DeJong]

I may need to run a test to settle this….

[jono]

I'd be interested to see a measurement of how much water is actually in your average brake line. I'm assuming (probably incorrectly) that water ingress into a brake system is probably similar from brake to brake, regardless of whether it's DOT or mineral oil. So if you took a sample after, say, a year of riding, how much water would be in the system?

My guess is that it'd be far less than 3.7%. Most cars have an open braking system, so the fluid is presumably a lot more susceptible to absorbing moisture. But most car manufacturers only recommend changing brake fluid every few years (and plenty of people go way longer than that without any massive negative consequences). If car brake fluid was hitting the 3.7% saturation point, I would think manufacturers would recommend a more frequent flush. So based on that, it seems pretty unlikely that bike brakes with their closed system are getting anywhere near 3.7% water content.

I'll play Lumberg and go ahead and disagree with a couple of those assumptions.

One, fluid that absorbs moisture from the air should attract a lot more of it than a purely (?) non-polar oil that is basically hydrophobic. So I'd expect more water in a DOT fluid system, and--up to the actual saturation point--all of it to be in solution with the fluid, so no chance to just boil at some low temp in the calipers.

Second, most cars will be fine for most drivers at the DOT spec saturation point and probably a bit past, because the spec purposely sets a minimum wet boiling point that is acceptable for most applications. The absorption rate should fall as the fluid gets "wetter," too. Better to flush it more often? Sure, but just like with bikes, it's the air that's gonna make the pedal soft. And there won't be a low point that pools water and heats it up, as would happen on a bike with mineral oil and water (if you could actually get significant water into the bike's system, but you'd probably have to pour it in to do that).

But I await deJong's testing, because it all seems like WAGs. Well, that and if puregravity isn't gonna come back, somebody really should pick up that torch, at least until there's snow.

[Roxtar]

I would tend to believe the improvement you notice is far more due to the new bleed than the new fluid.

[XXX-er]

I would agree that brakes feel better after a bleed cuz you ^^got rid of the air in the system, as oposed to new fluid really did anything one could feel

I had avid juicy 7's that I never needed bleeding altho I didnt use the bike much in the 14 yrs I owned the bike and i think here are many instances of 20+ yr old bikes with old hayes that were never bled