Ducati Sporting Club UK > Model Specific - Discussion, Technical Problems and Help > 998 / 996 / 916 / 748 > Rev counter not right. Air in brake system

[polarexpress]

Originally Posted by DEMON

Dot 4 is glycol based fluid, absorbs water. dot 5.1 is silicon based and does not absorb water...so any water / moisture found in the dot 4 system is usually absorbed into the fluid. With dot 5.1 it does not absorb water, therefore any in the system accumulates at the lowest point, ie your calipers and causes corrosion there!!

Thanks, demon. Perhaps I shouldn't run DOT 5.1 after all.

The reason I do so is because I assumed the DOT 5.1 fluid would eliminate the problems associated with water since it doesn't absorb moisture but I guess I misunderstood the whole concept.

I replace the brake/clutch fluid at least twice every season but I guess water could remain at the bottom of the calipers since the bleeder nipples are positioned at the top of the caliper instead of the bottom. Even if you connect a vacuum hose to the bleeder nipple you probably won't succeed in emptying the calipers completely.

Is there some indication of water still being trapped in the system? How about discolouration of the fluid? Will that still occur with no water whatsoever in the system?

Why does the fluid darken over time anyway?

And other than DOT 4 being hydroscopic what else is different when comparing DOT 4 and DOT 5.1? Is DOT 4 capable of handling the same amount of heat as DOT 5.1?

Thanks! And sorry for this partial threadjack.

[DEMON]

No prob with the hi jack…info is valuable to all..

Silicon based brake fluid has a higher boiling point than that of glycol based fluids (dot 3 and dot 4), but not dot 5.1

Dot 2 – not used any more, but was castor oil based
Dot 3 dot 4 – polyethylene glycol, and mineral oils, esters, ethers based

Dot 5 – silicon based
Dot 5.1 – glycol based…yes that’s right glycol

Glycol based brake fluid is approx 2 times less compressible than the others, so gives a better feel.
Lets remember one important fact regarding braking systems with fluid inside it…you are not compressing the fluid !! you are applying a force at one end of the system, which applies another force at the other end.

The reason brakes feel spongy, is the inclusion of air, which is compressible.


Boiling points are as follows :-
DOT 3 205°C
DOT 4 230°C
DOT 5 260°C
DOT 5.1 270°C

If you are using dot 5 brake fluid and want to remove all the water, you can, but its not easy. The simplest way is to remove all the fluid as best you can, then apply a vacuum pump to the lines and calipers. This lowers the boiling point of the water and turns it into a vapour. This can then be sucked out of the system.
obviously you may encounter probs with pistons being sucked back against their stops, and air being drawn into the system, depending on the mechanical state of your system !

Obviously you don’t see many diy’ers doing this to their bikes as its specialised.

My opinion is to use dot 5.1.
What a lot of people do when they buy a tin of brake fluid is shake it up on their way home..thus mixing the fluid with the vapour in the space above the fluid. They then put this straight into the system and wonder why their brakes are spongy !

Best thing to do is when you get the fluid back home, let it stand overnight and settle, allowing the bubbles to escape.

[DEMON]

The brake fluid can darken for a number of reasons..exposure to UV light, degradation from use, oxidation from heat from your discs, contamination by moisture etc.

[twpd]

If your brakes go spongy after using them hard a few times then there is a simple cause..it's either air or water or both. Nothing else.

Sometimes it helps to leave the bike on its sidestand overnight with the reservoir cap off and the lever clamped to the bar. It's a common trick. Also I often tap the brake lines when bleeding them - helps the air rise back up the line to the reservoir.

[polarexpress]

Thanks a lot for the info, demon and twpd!

I'll continue using DOT 5.1 after all, then. I guess the slight discolouration I see over time is the result of water and UV-light but I'm not going to worry about it as I am replacing the "old" fluid with fresh fluid regularly.

[twpd]

FWIW, dunno if it helps you feel better but, I change the fluid in my race bike after every race meeting. It's cheap after all - not suggesting you do that but once every 6 months is a good measure for keep everything tip top.

[KeefyB]

Some great info on this thread.Keep it up fellas.

[Strada916]

Originally Posted by twpd

Sometimes it helps to leave the bike on its sidestand overnight with the reservoir cap off and the lever clamped to the bar.

That is a good tip, but I would not leave the reservoir cap off overnight. That would allow moisture in the air to be absorbed into your brake fluid.
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