Brake Fluid Replacement and getting rid of absorbed moisture. - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 17 (permalink) Old Jun 4th, 2019, 4:55 am Thread Starter
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Brake Fluid Replacement and getting rid of absorbed moisture.

I have M3 BMW car and have replaced the brake fluid every couple of years. Brake fluid absorbs moisture (water) and water is heavier than brake fluid, so despite the changes, the water still sat in the bottom of the calipers and chewed out some very expensive front brake calipers. Lesson learnt...

The issue is is that most bleed nipples are at the top of the caliper, and so the heavier water, that corrodes the cylinders and pistons, does not get displaced on a fluid change.

So I have decided to do fluid changes on my Ducati's, but I don't want the fk up I had with the M3.

The process I have at the moment is:- (and this includes clutch)

bleed new fluid through with slave cylinders in position
remove the slave and spray exposed pistons (not pads) with Inox
push pistons back in fully and chock wide open with timber
cable tie slaves up so that the bleed nipple is at the bottom most point and Let SIT overnight
bleed in this position
refit slaves and re-bleed


IS there a better way to do this to get rid of the water?

Some will say strip and replace, but with my Ducati's and Zane Laverda's as well I'm looking at a total of, including clutches, 103 pistons. Not a financially feasible exercise.

Regards, Greg
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post #2 of 17 (permalink) Old Jun 4th, 2019, 6:40 am
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https://www.motiveproducts.com/colle...brake-bleeders

I use the motive pressure bleeder. Makes the process much cleaner and simpler.

I am under the impression that moisture becomes absorbed in the brake fluid, and will flush out with the old fluid. I don't think it pools in the calipers.

Regardless, a pressure flush will eliminate everything in the system, all contamination, and leave you with an air free, fresh fill.

This piece seems to support my understanding that water is absorbed into the brake fluid, and does not separate and pool.

https://www.carid.com/articles/why-i...uid-flush.html

While water in the brake system is the main reason fluid flushes are performed, the hygroscopic nature of brake fluid actually proves helpful in preventing other types of problems. For example, if brake fluid was not hygroscopic, water (which is heavier by weight) would pool together in pockets and gravitate to lower areas in the system such as the brake calipers. This would create a more serious corrosion problem, and those pockets of water would boil much sooner than when water is dispersed evenly throughout all of the fluid. more details on - https://www.carid.com/articles/why-i...uid-flush.html

Last edited by Pard; Jun 4th, 2019 at 7:08 am.
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post #3 of 17 (permalink) Old Jun 4th, 2019, 7:18 am
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As Pard points out, you have a misunderstanding about brake fluids. Absorbed water from the air does not separate-out and pool at lower points in the brake components.

The major brands of brake fluids consist of ethylene glycol, polyglycols, silicone fluids, and isobutyl alcohol. These constituents have the same hydrophilic characteristics due to their molecular O–H structure, which easily allows hydrogen bonding with hydromolecules, i.e., water. The hydrogen bonding is based on the O–H bond in the molecular structure, which permits dipole attraction of the other hydromolecules. In other words, the water molecules are chemically bound-up by the brake fluid and can’t separate out.

Further, brake system water content tends to reach a maximum of about 3%, which is readily handled by the corrosion inhibitors in the brake fluid formulation. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. Follow Ducati’s change interval recommendations.
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post #4 of 17 (permalink) Old Jun 5th, 2019, 4:13 am Thread Starter
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I would like to thank you both for correcting me.

If one does not learn something new every day, one is not living. What has happened with the M3 calipers then, is that I left the fluid in for too long, and the inhibitors have been depleted and allowed an increase in hydrogen(?). The M3 is stored and not driven. Fuel pumps are another issue....

Thankyou!

Greg
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post #5 of 17 (permalink) Old Jun 11th, 2019, 12:45 pm
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Could it be that the M3 was using DOT5 fluid? That's a silicone based brake fluid, and water will separate out (and probably sink to the bottom).

The "normal" brake fluids are DOT3, DOT4, and DOT5.1

I believe Ducati recommends DOT4.
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post #6 of 17 (permalink) Old Jun 11th, 2019, 1:45 pm
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I use 5.1 in all my bikes, but haven't had my Duc long enough to warrant a flush. Isn't 5.1 a better option for fluid?

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post #7 of 17 (permalink) Old Jun 11th, 2019, 3:24 pm
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DOT 5.1 is a lighter viscosity glycol-based fluid that was developed for use in ABS systems that need to cycle on and off quickly. It really has no advantage (in non-ABS systems) over the DOT 4 fluid recommended by Ducati. Also, DOT 5.1 fluid can have worse performance after absorbing moisture than some of the better DOT 4 fluids.

The DOT specification for DOT 5.1 brake fluid has a higher minimum dry boiling point/minimum wet boiling point (527F/347F) than the DOT 4 spec (446F/311F), but these are just minimum specs.

Several DOT 4 fluids exceed DOT 5.1 specs. For example, expensive Castrol SRF (590F/518F) is used by Formula One teams, ATE Type 200 and Super Blue Racing (536F/392F), and Motul RBF 600 (593F/421F). Golden Spectro Supreme DOT 4 (520F/367F) comes close.

The minimum kinematic viscosity spec for DOT 5.1 is 900 cst half the viscosity specified for DOT 4. Leakage of 5.1 past the seals has been reported and cured by using DOT 4 fluid.
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post #8 of 17 (permalink) Old Jun 11th, 2019, 3:26 pm
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Quote:
Originally Posted by Ghwallice View Post
I have M3 BMW car and have replaced the brake fluid every couple of years. Brake fluid absorbs moisture (water) and water is heavier than brake fluid, so despite the changes, the water still sat in the bottom of the calipers and chewed out some very expensive front brake calipers. Lesson learnt...

The issue is is that most bleed nipples are at the top of the caliper, and so the heavier water, that corrodes the cylinders and pistons, does not get displaced on a fluid change.

So I have decided to do fluid changes on my Ducati's, but I don't want the fk up I had with the M3.

The process I have at the moment is:- (and this includes clutch)

bleed new fluid through with slave cylinders in position
remove the slave and spray exposed pistons (not pads) with Inox
push pistons back in fully and chock wide open with timber
cable tie slaves up so that the bleed nipple is at the bottom most point and Let SIT overnight
bleed in this position
refit slaves and re-bleed


IS there a better way to do this to get rid of the water?

Some will say strip and replace, but with my Ducati's and Zane Laverda's as well I'm looking at a total of, including clutches, 103 pistons. Not a financially feasible exercise.

Regards, Greg
From what I have seen the bigger issue with motorcycle calipers is that, unlike cars, they do not have dust boots on the pistons, just the piston seals. So over time, a ridge of brake dust mixed with fluid residue will build up. This will work against retraction when you let off of the lever.

I do what you are suggesting each time that I replace the pads. Pop out each piston and thoroughly clean them using a microfiber cloth wetted with brake fluid. They slip right back in. This is an easy process using some zip ties on three pistons that remain in and a bit of pressurized air. The calipers bleed very easily.

Otherwise, I do a brake bleed each spring. The system volume is less and exposure to air is greater than in a car.

Have a good one.

tony b
2001 ST4 - Thumper
2013 Ural GearUp (sold)
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post #9 of 17 (permalink) Old Jun 11th, 2019, 3:30 pm
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Quote:
Originally Posted by mabrungard View Post
I use 5.1 in all my bikes, but haven't had my Duc long enough to warrant a flush. Isn't 5.1 a better option for fluid?
I tried DOT 5.1 a while back and it seems to shrink the seals just a bit. This caused my clutch master to leak a lot. I thought it was the seal wear. When a rebuild didn't work, I slapped on some 996 masters that started to do the same thing after a while. On a whim, I flushing in some DOT 4 and the leaks went away.

I'd say to stick with DOT 4. Some, like Motul Racing have a wet boiling point that higher than a lot of dry boiling points.

Have a good one.

tony b
2001 ST4 - Thumper
2013 Ural GearUp (sold)
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post #10 of 17 (permalink) Old Jun 12th, 2019, 8:07 pm
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I appreciate the education on DOT 4 vs 5.1

I learned something today!
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M. Brungard
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