Boiling rear brake fluid

[Scud]

I think it might be the non-exisitant lever return spring on the brake pedal. I put a spring on it and my brake dragging problems went away. I also like having a more solid feel to the pedal so I know I'm not accidenally slightly pressing it with my large, heavy boots.

I use the rear brake very little yet before putting the spring on I was wearing through them faster than the fronts.

 

Where, exactly, did you put the spring? (a picture would be nice if it's convenient for you)

 

I wonder if the brake could be "auto-engaging" when the bike goes over bumps. Could the pedal bounce enough to put just a bit of pressure on? This could explain why the spring helped so much. This could be my problem too, as I didn't think I was using the rear brake very much.

 

FYI - I don't think I boiled the fluid and I didn't lose pressure - just noticed that rotor was quite hot.

[GuzziMoto]

Guzzi's have more weight on the rear than most other bikes. Plus, V11's have a small rear brake for the size. Todd even had a plan at one time to offer a larger rear brake with a larger pad to go with it. I don't know if he ever went anywhere with it.

If you use the rear brake much on a V11 I can see having issues. I use the rear brake very little, and I have no issues.

However, I think you need to determine whether it is just fading or if you are actually boiling the fluid. If you boil the fluid the brakes will fade and they don't fully come back until you bleed them. If they fade but after cooling they come completely back I don't think that is likely the fluid boiled. Boiled fluid needs bleeding to get braking fully back to normal.

It is also possible you are exceeding the operational temperature of the brake pads. I would suggest if that is the case you should switch to a full metallic, perhaps an HH pad. EBC should offer such a pad for the rear brake of a V11, but I would consider trying either stock Brembo pads or a different brand. EBC are not exactly the top line brake pad company. That said, they do offer some good high friction full metallic pads. But consistency over a wide operational temperature range is not always their strong point.

[Cabernet]

It does not fade, it falls off a cliff.
 
One corner it is there and fine. The next the peddle goes straight to the stop. No noticeable back pressure or anything. Stop and the caliper is to hot to touch, the peddle to the stop with a little finger and but I wouldn't have said the disc was overly hot. The occasion "tink!", but not fully cracking away like some do. Wait 5 minutes or stop using the brake whilst on the move and it comes like it's never been away.
 
My school boy physics deduces, "on the last application the fluid rises above the boiling temp of the fluid at atmospheric pressure, but as the fluid is under pressure the boiling point is elevated (like water in a pressure cooker) so the brake works fine for the duration of the braking. When the brake is realised, the drop in fluid pressure allows the fluid to vaporise. Then on the next brake application the vaporised gas can be compressed so the brake fails. Let the system and fluid cool and the vaporised fluid condenses back to a liquid and all works again. As I have experience this in the Alps and Black forest hills where the boiling point of water is lower, this would also explain the Breva experiencing the same problem less frequently, assuming brake fluid boil points are lower at altitude also.
 
Having looked into it, I see three possible alternatives I/we can try whilst staying with the OE disc and caliper.

• Use a racing brake fluid. Most manufactures boast 300+C boiling points. I have ordered "EBC BF307+" today..

• Use a ceramic backed pad to insulate the piston from the pad heat. DP brakes offer these..

• Fabricate a heat sink. There may be room for a shims to be placed between the pad and the piston, the same shape at the pads that extends out of the caliper toward the wheel spindle. However these may vibrate and cause brake squeal and or pad rattle (knocking the pistons back into the calipers to far, require pump up to operate)..

[Scud]

A friend of mine put something like this on his KTM rear brake - said it helped. Maybe something like this can be adapted?

 

http://slavensracing.com/shop/brake-caliper-cooler-ktm-husaberg/

[JBBenson]

I sort of don't get this thread. So, boiling = fade? Brakes fade for a few reasons. I have never "boiled" my brake fluid as far as I know.

 

Having read all of the previous posts, it doesn't make sense that the rear brake would "boil" and the front one wouldn't. After all, the front brake is under a lot more stress than the back, with the weight transfer to the front under any kind of braking at all. It's not like the back rotor isn't being cooled either, if that's the theory. I doubt the diameter of the rotor plays a role either. They both get f-ing hot.

 

Not trying to be a crank. Can someone explain it to me?

 

I try to use the brakes as little as possible when riding, even if the riding is "spirited". I always aim for smoothness as it often raises my average speed anyway, and feels better and is more satisfying that all that lurching and wobbling and squatting and diving that I see all the time.

 

Maybe I am just a crank.

[Lucky Phil]

 

If you're boiling the fluid in the rear brake of a V11 then you have a problem with riding technique. 

Then why after thirty years and near 900,000 miles of motorcycling has it only shown up on a Guzzi and predominantly on a V11? 

 

Same reason I've had the problem with racers, because in the heat of battle they unconsciously drag the rear brake. Usually solved for them by and extra return spring or rubber band. Racers have enough on their hands already so I cut them some slack. 

Ciao

[GuzziMoto]

I also am not sure the fluid is actually boiling. My experience with boiled brake fluid is once it boils it does not come back to 100% normal without bleeding.

Clearly something is going on, but I would be surprised that the fluid is boiling. I don't think it is the only possible explanation.

The only way I could see the fluid boiling would be if you are dragging the brake constantly, and the discoloration of the brake disc would be a clear sign of the heat being generated. And, as I said, I don't think it would come back to normal in a short time. It may be overheating the brakes, but boiling fluid causes a chemical change in the fluid that does not just go away when it cools down.

[gavo]

Brake fluid is Hygroscopic (it absorbs water from the atmosphere) this is the reason it should be replaced regularly.

When the fluid gets hot, because of the moisture absorbed the boiling point is significantly lower, you get some vapour in the fluid and this compresses ,result, no brakes

the lever goes to the stops.When it cools its all fluid again and won't compress the brake pedal returns.

Just basic science a vapour can be compressed a liquid cannot.

[Cabernet]

Well it feels just like someone had opened a bleed nipple. Afterwards, it feels lime nothings happened.

It happens more whilst touring spiritedly, than balls out going for it. Like I said earlier, I have change my style at track days and can bias the front for 20 mins in the hour. But on a sustained Sunday run is when it gives up, on tight twisty British B roads (Euro D) with elevation changes and not the smoothest of surfaces I favour an easier pace, lifting and coasting on over run, fettling the pace with the rear to sweep through bends rather than brake, point and shirt.

[Cabernet]

So, boiling = fade?

I take brake fade to be when the disc and pads get so hot that they can no long absorb and disperse the heat energy created. This gives the feel of a solid system and as if an ever increasing amount of oil is being pumped on the pads as the braking power fads away.

I take brake boiling as when the heat transfers through the pads and pistons to the fluid which turning gaseous preventing the fluid pressure building in the system to operate the pads.

[Cabernet]

Brake fluid is Hygroscopic (it absorbs water from the atmosphere) this is the reason it should be replaced regularly.

When the fluid gets hot, because of the moisture absorbed the boiling point is significantly lower, you get some vapour in the fluid and this compresses ,result, no brakes

the lever goes to the stops.When it cools its all fluid again and won't compress the brake pedal returns.

Just basic science a vapour can be compressed a liquid cannot.

Yes.

Now whether water has got in the system or the fluid is simply over heating, I hope fresh racing brake fluid will improve things. I'll keep you posted.

[Lucky Phil]

As Guzzimoto pointed out, if it's overheating enough to boil the fluid the disk will be blue.

Pedal going randomly full travel sounds like primary master cylinder seal to me

Ciao

[Cabernet]

As Guzzimoto pointed out, if it's overheating enough to boil the fluid the disk will be blue.

Pedal going randomly full travel sounds like primary master cylinder seal to me

Ciao

Ok, why would a seal that id remote from the "action" fail and recover in the ways described?

[czakky]

Do you have any play at the rear wheel? Disc?

[Cabernet]

Do you have any play at the rear wheel? Disc?

I have experience it on both my rear wheels.

I have changed our the bearing spacer and bearings on both.

Both rears have their own disc too.

I believe I can therefore eliminate all the rotating parts.