, i have seen this before on race bikes my mate GSXR 1100 front brakes did it on a track day ...but also on road machines usually front brakes ......in fact it happened to me last week on the mountains in Petchabun but it was my rear brake heavy use on the down ward mountain bends for 30 miles suddenly on one nasty bend it failed completely in abend covered in sand and gravel its a nasty feeling when the brake pedal goes all spongy and the lever goes to nothing......I brought the bike to a contolled halt and inspected the lever and brake all was OK ????? on the next bend it was fine but during the whole hard trip it happened 4 times .Now back home i have dismantled it completely and found nothing wrong ,,,i reckon the cause is overheating causing a temporary loss of pressure but as the system cools the pressure comes back ,the pads and fluid have now been replaced and i will get a rebuild kit for the caliper and master cylinder o rings
What do u guys think????
So you're saying you lost all rear brake?
What kind of bike?
It certainly could be heating the fluid enough to lose pressure. When I had the 996, I found that with the rear brake reservoir as close to the headers as it was the pedal would go spongy and useless after a while, but would return to normal after everything cooled down.
Sounds like old brake fluid that had absorbed a bunch of water got hot enough to boil. Vapor (steam) doesn't transmit pressure nearly as well as liquid.
Any idea how old the brake fluid was?
Quote from: Buckethead on February 19, 2011, 09:03:44 AM
Sounds like old brake fluid that had absorbed a bunch of water got hot enough to boil. Vapor (steam) doesn't transmit pressure nearly as well as liquid.
Any idea how old the brake fluid was?
+1.
Almost all brake fluid should be replaced yearly, and in picking a brake fluid for
street use, the wet boiling point is the one you want to pay most attention to. (By contrast,
track use would focus on the dry boiling point since it should be fresh / water free when you go out)
Good post by this Shazaam character on .MS comparing the various common high performance brake fluids for moto's, i shall copy with a link as well:
QuoteBest Brake Fluids
(http://www.ducatisportingclub.com/images/dsc/shazaam/BrakeFluidsBP.jpg)
Castrol SRF is the best, hard to find, four-times more expensive than the others, and used by every Formula 1 team in existance because it's boiling point drops very little as it absorbs moisture.
Motul RBF 600 is next best and reasonably priced. Same dry boiling point and the next-highest wet boiling point. It tends to absorb water more quickly than the others so it seems to be best suited for racing applications where the fluid is changed very frequently. Keep in mind that any fresh dry fluid will be better than wet Motul because it will have a higher temperature boiling point.
This suggests to me that ATE is the best all-round brake fluid.
BMW recommends this brake fluid for their street cars because it, like Castrol LMA, absorbs moisture very slowly. The advantage over LMA is that ATE has a much better wet boiling point. You can use this in your street bike and forget about it for a year or so. An excellent choice for a weekend track bike which also sees regular street duty.
ATE Super Blue and ATE Type 200 and are the same fluid. The theory is to alternate them and you can tell from the color when you have flushed all the old fluid out. In practice however, once a can is opened it should be used-up quickly. It's not a good idea to save fluid from an opened can for color change reasons.
link: http://www.ducati.ms/forums/57-supersport/96571-ate-racing-blue-brake-fluid.html#post932640 (http://www.ducati.ms/forums/57-supersport/96571-ate-racing-blue-brake-fluid.html#post932640)
And in different place:
QuoteDOT 5.1 Brake Fluid
Before you buy, read the container.
The DOT specification for DOT 5.1 brake fluid has a higher minimum dry boiling point/minimum wet boiling point (527°F/347°F) than the DOT 4 spec, but these are just minimum specs. Some DOT 4 fluids exceed DOT 5.1 specs. For example, expensive Castrol SRF (590°F/518°F) is used by Formula One teams, ATE Type 200 and Super Blue Racing (536°F/392°F), and Motul RBF 600 (593°F/421°F). Golden Spectro Supreme DOT 4 (520°F/367°F) comes close.
So DOT 5.1 fluid is not necessarily better, particularly after absorbing moisture. Most of us don't change our brake fluid very often so wet boiling point numbers are more important. On the track, brake fade is the main concern so we change the fluid more often and use a higher dry boiling point fluid. Generally, the higher the dry boiling point, the faster the fluid absorbs moisture from the air, Castrol SRF and the ATE fluids excepted. Note that ATE Type 200 and Super Blue are the same fluid, the theory is to alternate them so you can tell when you have all the old fluid out.
If you don't change your brake fluid once a year or so, the absorbed moisture in the fluid corrodes expensive brake bits. So, do it this winter.
Castrol SRF is the best, hard to find, four-times more expensive than the others, and used by every Formula 1 team in existance because it's boiling point drops very little as it absorbs moisture.
Motul RBF 600 is next best and reasonably priced. Same dry boiling point and the next-highest wet boiling point. Still, any fresh dry fluid will be better than wet Motul because of a still higher boiling point.
ATE Super Blue and Type 200 and are the same fluid. The theory is to alternate them and you can tell from the color when you have flushed all the old fluid out. In practice however, once a can is opened it should be used-up quickly. It's not a good idea to save fluid from an opened can for color change reasons.
Just to clarify:
DOT 3 and DOT 4 fluids are compatible with DOT 5.1 - but are NOT compatible with silicone-based DOT 5. DOT 5 brake fluid is dyed purple to warn against accidental mixing.
The use of the 5.1 designation is really confusing.
5.1 is a lighter viscosity glycol-based fluid with that was developed for use in ABS systems that need to cycle on and off quickly. It really has no advantage over the DOT 4 fluid recommended by Ducati.
link: http://www.ducati.ms/forums/56-superbikes/31383-clutch-fluid.html#post294709 (http://www.ducati.ms/forums/56-superbikes/31383-clutch-fluid.html#post294709)
additionally,
if your brake system is getting hot, make sure there is sufficient free play in the rear lever, lest it gets hot - things expand - pad drag begins which generates more heat - which causes more expansion - which causes more pad drag - cycle continues until eventually you have a cooked rear brake caliper, cooked rear brake pads, and cooked rear brake rotor, and typically a locked rear wheel.
so. poke around a touch and make sure everything is tip top.
+1 on it likely being old/wet brake fluid.
I had it happen on my bike several years ago on a similar road as the OP described.
New fluid and all was fine.
From what I see in my shop, this is with the brakes cold:
SBK's tend to have the most problems with the rear brake going soft.
Appears to be the combination of cooking the master and the SBK style hose and underhung caliper mount.
S*R Monsters are next worst on the soft pedal 'affliction', as they've got the SBK style hose and underhung caliper mount.
Some of those had defective master cylinders, seemed to be mostly S2R800's.
Then, the few DSS Monsters with the underhung caliper.
DSS Monsters with top mount caliper almost never get a soft pedal.
Have seen a couple of defective units on them as well, '05 - '06 vintage black master cylinders, like the S2R800's.
Quote from: Speeddog on February 19, 2011, 10:12:16 AM
DSS Monsters with top mount caliper almost never get a soft pedal.
Have seen a couple of defective units on them as well, '05 - '06 vintage black master cylinders, like the S2R800's.
Hmm... Similar to the black master cylinder on my 2006 Sport 1000, perhaps? I bleed it til it's solid and a week later it's 'meh.'
Thanks for all your comments it confirms what i suspected .
I have inspected all the parts of the S4 rear brake fully dissasembled it all is OK there is lots of free paly as I regularly check my bike, the brake fluid was only one year old it has been replaced ,we never get winter here its always between 70-105 farenheit but high humidity
I was on a very mountainous road for over 100 miles hundreds of bends lots of use of rear brake and it must have gotten very hot .
Very likely.
They don't leak fluid, but seem to somehow ingest air.
Quote from: Buckethead on February 19, 2011, 10:15:34 AM
Hmm... Similar to the black master cylinder on my 2006 Sport 1000, perhaps? I bleed it til it's solid and a week later it's 'meh.'
Quote from: Speeddog on February 20, 2011, 10:07:55 AM
Very likely.
They don't leak fluid, but seem to somehow ingest air.
Fookers. >:( Thanks. [thumbsup]
I am using MOTUL RBF 600 racing brake fluid as its the highest boiling point stuff available here.
Enough constant braking will eventually boil any brake fluid.
(http://i298.photobucket.com/albums/mm276/hlep2890/lowergear.jpg)
Quote from: jerryz on February 21, 2011, 03:24:50 AM
I am using MOTUL RBF 600 racing brake fluid as its the highest boiling point stuff available here.
Actually, you can get Motul RBF 650 in the States, but it actually has a lower wet boiling point than RBF 600, so you're better of with the 600 unless you change it very often. If you're racing, the 650 MIGHT be worth considering....just FYI.
Cloner
ABQ, NM
Quote from: Cloner on February 21, 2011, 07:54:57 AM
Actually, you can get Motul RBF 650 in the States, but it actually has a lower wet boiling point than RBF 600, so you're better of with the 600 unless you change it very often. If you're racing, the 650 MIGHT be worth considering....just FYI.
Cloner
ABQ, NM
He lives in Thailand. A higher wet boiling point may be somewhat desirable.
+1. if you read that post from shazaam, the most important tidbit that most people might not immediately consider is the wet-boiling point being most important in street bike applications. Dry boiling point is important for track bikes / race bikes where you put fresh fluid in all the time, but where it will be sitting for awhile, free to absorb moisture from the air, the wet boiling point becomes more important.
So, choose the brake fluid with a good wet-boiling point and one that is 'less hygroscopic'. Good luck finding much quantitative data on the latter, and if you do - please show me.