Boiling rear brake fluid

[GuzziMoto]

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.

Once the fluid boils, the water that may be in the fluid can turn to steam, plus other components of the brake fluid turn to gas. When the system cools off, most of the steam would revert to water but the other components that turned to gas stay as a gas. They do not return to being part of the brake fluid. The result is that the brake do come back but not quite back to 100% normal.

[guzzi323]

 

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)

 

 

This is nothing at all fancy. I just cobbled it together with what I had on hand.

Google's new, annoying system make it hard to embed images but a pic of it is here..

 

This solved my brake heating problems. I hope it helps.

 

johnk

[Scud]

John - 

 

Thanks for that. I took a screen shot and uploaded it.

 

 

That's a clever spot for the spring. I was imagining something on the front that pulled the lever up; your solution seems so obvious now.

 

I think I might use the rear brake accidentally sometimes - just by letting my foot get lazy. Or that I might use it unconsciously sometimes - carryover of dirt bike habits. A stiffer pedal should "cure" me of those habits. Anyway - I just had this hot brake problem once that I am aware of.

[Cabernet]

 

 

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)

 

 

This is nothing at all fancy. I just cobbled it together with what I had on hand.

Google's new, annoying system make it hard to embed images but a pic of it is here..

 

This solved my brake heating problems. I hope it helps.

 

johnk

 

That is neat.

[theoneandonly]

Did you check your main brake cylinder? Maybe the back flow port is partially clogged and pressure falls to slow, Enough clearance (5-10mm) between brake lever rod and piston.

 

there have been problems with rr master cyl, pull it apart and check .

[dl.allen]

wow that is a really cool spring idea.  I will be on the lookout for a spring.  I think my problems may have started from my foot inadvertently hitting the pedal. 

 

Havent heard of the intermittent pedal from heat only fade.  I would get new fluid in there asap, check for rotor warpage, check master cylinder for return etc.

 

The spring idea is the best Ive seen in a long time!  kudos!  

[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.

Once the fluid boils, the water that may be in the fluid can turn to steam, plus other components of the brake fluid turn to gas. When the system cools off, most of the steam would revert to water but the other components that turned to gas stay as a gas. They do not return to being part of the brake fluid. The result is that the brake do come back but not quite back to 100% normal.

 

Which components would they be? any parts that are liquid at say 20 deg c and then become gaseous at temperatures above its boiling point will revert back at the lower temp.

Boiling the fluid is not a distilling process unless you remove the gases.

[guzzi323]

 

 

 

I think I might use the rear brake accidentally sometimes - just by letting my foot get lazy. Or that I might use it unconsciously sometimes - carryover of dirt bike habits. A stiffer pedal should "cure" me of those habits. Anyway - I just had this hot brake problem once that I am aware of.

That was my problem. I like to place my brake pedal so they are right under my boot in normal riding and it was easy to be unaware that I was slightly pressing it. With this spring (I think it's an old cluch spring from my DR 350) it takes noticeable pressure to move the pedal.

[xbr5]

Thanks for the thread guys.  I've had my V11 a few months but mostly ride my TDM for the commute to work.  One day I took the V11 and when I got to my exit of the motorway on my way home, I found the back brake was completely ineffective.  I too realised that I may have been inadvertently been dragging the back brake (different bikes, different lever positions), causing it to "fail".  After a couple more miles, with my boot well away from the lever, the pedal came back to life.  I will also look for a decent spring to add to the pedal.