total temporary front brake failure on 1985 bmw krt100

[pinhead]

I was riding with a passenger (about total 320 lbs wt for both of us) and was going down long mountain grade dropping 2800 ft in less than 5 miles, lots of sharp corners and switchbacks.  I noticed as I was reaching end of downgrade, my front brake was requiring more travel to brake.  Soon after that complete failure!  Compression of brake handle completely and no braking what soever.  Observed brake fluid level and no change in level while this occurred.  Stopped using rear brake pulled over and had no front brake whatsoever.  Was in mountains 60+ from home and determined that I would ride very carefully home on my rear brake only.  Got down the road about 4 or 5 miles and checked front brake and it was working normally again.  No spongy feel, no air in brakes no visible loss of brake fluid. 


What happened?  Living in mountains and experienced normal brake fade and this was more extreme and complete.  Was wondering if the brake fluid temporarily boiled away from excess heat?

[jjs1234]

Change the fluid. Could have air/water in the lines is my guess. I had something similar on my car. But I would check the plunger too.

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[Elipten]

I'll bet that was scary!

How old is that fluid?

Make sure you use the correct type of brake fluid!


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[Chaos]

bleed / flush the brakes.  Brakes got hot enough to boil the water in the fluid, steam in the lines turned the brakes to mush, brakes cooled down, steam condensed and brakes returned to normal.  No harm done. 

[Laitch]

Here's some fun information from a maker of things for you to buy.

[Martin]

+1 Chaos you need to change your brake fluid regularly, brake fluid is hygroscopic which means it will absorb moisture from the air. I change mine every two years.
Regards Martin.

[pinhead]

brake fluid was clean, changed less than 3 years ago.  I have worked on bikes and vehicles with brake fluid so old it was almost black and loaded with particulates from corrosion.  Yet never seen a complete brake failure on such vehicles.  I don't think there was a problem with brake fluid.  but I will definitely change and use high temp fluid.

[Laitch]

I don't think there was a problem with brake fluid.  but I will definitely change and use high temp fluid.

Did you read the link about thermal transfer and brake pad composition reacting to friction heat?

[pinhead]

No I have not read that link.  But if this bike can't stand the brake demand I put on it, it is a scary thought for those who put greater demands on its braking system.  Is this bike raced (not drag racing) anywhere, or was it ever?  Surely if heat was a problem it would show up there. 


Over the years I have repaired cars and trucks that have had sticking pads and literally have burned down the pad and disc when the driver failed to notice something wrong, but no brake failure.  There is something really peculiar here.  I was going to change the brake fluid today but I decided not to. I am going to try and overheat these brakes again and see if the same failure results.  If I can't reproduce the same result, I will replace all pads, inspect and then :dunno2:

[Filmcamera]

You need to change the fluid.  The symptoms you described when the brakes failed and then came back is exactly what happens when the fluid is old and has water in it.  If you try to recreate the issue you will have no problem doing so.  The water in the fluid will boil and you will have no brakes.  Let's hope you can safely stop again second time.  Not sure I would purposefully create a brake failure when various people have told me why it is happening and how to fix it but it is up to you.

[Laitch]

No I have not read that link.  But if this bike can't stand the brake demand I put on it, it is a scary thought for those who put greater demands on its braking system.  Is this bike raced (not drag racing) anywhere, or was it ever?  Surely if heat was a problem it would show up there.   

You need to change the fluid.  The symptoms you described when the brakes failed and then came back is exactly what happens when the fluid is old and has water in it.

Read the link, pinhead.

Racers do not apply their brakes for extended intervals like riders on long, steep mountain descents. Undoubtedly, brake fluid plays a critical role and I believe somebody like you with an apparent thorough knowledge of brake performance would have inspected the cylinders' performance, bled the brakes and used the correct type of fluid previous to riding in the mountains.

Disc and pad thickness and composition also play roles in heat transfer to inhibit glazing of the pad surface. Glazing causes lack of friction and brake failure.

Mindful technique used in braking on descents is also important.

[Chaos]

Read the link, pinhead.

I thought your were being horribly rude and insulting until I realized you weren't.   :hehehe

[milq]

bleed / flush the brakes.  Brakes got hot enough to boil the water in the fluid, steam in the lines turned the brakes to mush, brakes cooled down, steam condensed and brakes returned to normal.  No harm done.

What he said, classic example of the situation. Both DOT 3 and 4 brake fluids are hygroscopic (they absorb water). The wet (a certain amount of moisture is present) boiling point of DOT 4 is around 310 degrees F if memory serves, where fresh "dry" fluid is more like 425.
 Add to that, the pads lose effectiveness (but won't recover) when they get hot and glaze over. But my money is on heat transfer to the fluid.

[Laitch]

Add to that, the pads lose effectiveness (but won't recover) when they get hot and glaze over. But my money is on heat transfer to the fluid.

Heat can produce pad resin gasses that cause fade and then the pads recover when cooled.

Was the lever travel soft for a while, pinhead, or did the feel of resistance in it just disappear?

[K1300S]

hard lever with no braking is pad fade.
lever to the handle with no braking is boiled fluid.

never drag the brakes on long downhills.   stock brake systems can't effectively dissapate that much heat.  downshift for engine braking and use the  brakes in short spurts to allow long cooling periods.

plus...don't use cheap brake pads.  get good high temp rated HH pads.  and don't use cheap brake fluid.  get higher temp stuff like ATE Type200.  this is especially true if you KNOW you will be riding in the hills a lot.

[Chaos]

and drill some cooling hoes in them rotors...

counter rotating hole pattern on left and right rotors if effective.

[pinhead]

Talked to service at A and S bmw dealership in Roseville about the problem and they have heard of this before.  Has to do with quick altitude change and, I assume, the inability of the master cylinder ventilation system to cope with the quick atmospheric pressure change.  And of course, heat in brake fluid from semi continuous braking.  Did not mention water in the brake fluid.  Suggested replacement of master cylinder.


Out here on West Coast, we have continuous downhill grades which maybe some of you have never or seldom ride.  I do most of my riding either in the mountains, up to 11000 feet and down in the summer.  My RD350 I have ridden much harder down steep grades with no such failure.  Didn't have a passenger but RD350 has only one brake disk, not 2 like bmw. 


Holding front brake on for a mile or 2 while driving on a flat highway should heat brakes up to higher temp than what was encountered on my trip.  Be interesting to see if it repeats. 

[The Mighty Gryphon]

Talked to service at A and S bmw dealership in Roseville about the problem and they have heard of this before.  Has to do with quick altitude change and, I assume, the inability of the master cylinder ventilation system to cope with the quick atmospheric pressure change.  And of course, heat in brake fluid from semi continuous braking.  Did not mention water in the brake fluid.  Suggested replacement of master cylinder.

I love it!

Why just bleed the brakes when you can sell a new master cylinder, too?

WTF is a master cylinder ventilation system???  Is it connected to the blinker fluid reservoir?

One thing not mentioned is that higher altitudes lower the temperature at which water boils, so of course, your brakes will fade sooner at high altitudes.  At 5000ft. the boiling point will be about 10% lower than at sea level.

Flush the goddam system.

[K1300S]

Talked to service at A and S bmw dealership in Roseville about the problem and they have heard of this before.  Has to do with quick altitude change and, I assume, the inability of the master cylinder ventilation system to cope with the quick atmospheric pressure change.  And of course, heat in brake fluid from semi continuous braking.  Did not mention water in the brake fluid.  Suggested replacement of master cylinder.

LOL!! love a dealer with a sense of humor!   

flush the system with good fluid.  put new GOOD brake pads in.  brake properly on long downhills. done.

can of ATE - $15
two sets of EBC HH pads - $45
changing use of lever - free
increase in braking safety - priceless

[K1300S]

counter rotating hole pattern on left and right rotors if effective.

two left feet....

[Elipten]

That dealer is full of bull IMHO.  I have ridden numerous London downhill mountain runs in my life and never had an issue.  I would never ride the brake for two miles.  Engine braking for down hill runs is mandatory in a car, truck or motorcycle.

Start with bleeding the brakes with fresh fluid.  Then pull and check pads.  Replace as necessary.  Then go test.


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[Mickster]

Out here on West Coast, we have continuous downhill grades which maybe some of you have never or seldom ride.

   :hehehe

[Laitch]

   :hehehe

It's hell out there, I tell you. Hell!

[Mickster]

It's hell out there, I tell you. Hell!
[/quote
Yes, if you go downhill continuously you will surely end up in Hell...   :dunno   :mbird

[Laitch]

aint never seen any continuous downhill grades... must getts deep in there... and then go deeper... continously deeper...

It's getting continuously deep here, too! End Times!