WR rear brake fluid boils

The last two times I went riding my rear brake fluid boiled which resulted in a loss of brakes. I don't think I am a brake drager, can anyone offer any suggestions to fix this problem. I completely drained and replaced the fluid each time it boiled. I am thinking about adding to the capacity of the brake resevoir and or adding a wave rotor. Please help, Thanks, Dirtmizer.

Look into a brake resivoir extender.

How will brake reservuar extender gonna help, when brake fluid boils in the calipher? Maybe try with better brake pads. I use Carbone Lorrane and I swear by them!

Adding capacity will just allow you to boil more fluid. You need to find the root of the problem to fix this. Check a few things first to troubleshoot it like: warped rotor (replace), sticking caliper piston (clean), sticking master cylinder (clean), sticking brake pedal pivot shaft (clean/lube), missing or weak brake pedal return spring (replace).

These are probably the most likely causes but there is another you might not be thinking of too. Do you have new boots? If so they wont have the feel they normally do when broken in well. I recently got a new pair of boots and noticed I could no longer "feel" my rear brake until it locked up. I didn't realize that I was dragging it and I had to make a consious effort to keep my foot off of the brake pedal. Just something to think about :thumbsup:

Is your reservoir retainer broken and getting too near the exhaust pipe ?

One issue may be the fluid your using. Make sure your using the proper, high quality, fluid.

I know Honda CRF's and some KTM's had issues with boiling the fluid due the the new rear reservoir design. Ride Engineering and Zip Ty racing as well as others I suppose have come up with a extended reservoir. Higer fluid capacity makes it harder to boil the fluid because there is just more of it to have to heat to the boiling point. They also provide a larger surface area for built up heat to dissipate. This quote might give you a better Idea on how they work. This came from the Zip Ty site.

"Zip-Ty Racing Products introduces the Brake Reservoir Extension for KTM dirt bikes. This is the solution to the problem of KTM’s integrated master cylinder/reservoir brakes. By increasing the brake fluid volume in a finned environment (increased surface area), you dramatically reduce the likelihood of boiling your fluid – thereby preventing brake fade and restoring excellent modulation. Zip-Ty’s Reservoir Extension features: 6061 T6 Aluminum construction and easy Installation – includes o-ring."

Since the fluid boils at the caliper like Sabin said, how does the larger reservior help? You aren't circulating the fluid.

Dude all I know its that some of the new master cylinder/reservior setup's have had this problem.

As far as where the fluid boils I don't know. All I know it that these systems have worked in reducing the amount of heat in a system. There by reducing or eliminating brake fade. They are also used by a lot of riders who are a lot faster then I'll ever be.

But as someone here has already stated. Check the overall system and make sure that there is not another reason for this problem to begin with.

Since the fluid boils at the caliper like Sabin said, how does the larger reservior help? You aren't circulating the fluid.

Remember that heat dissipates. Very important. So it seems to me that as the heat builds up it will spread. Now if the heat spreads into a smaller volume of liquid, in this case break fluid, It heats up faster.

Since the fluid is the force that drives the pads on to the rotor the fluid absorbs much of the heat generated. The other components of the system absorb the heat as well but it's the fluid that is the driving force in this system. The fluid can only take so much before it fails. Once the fluid fails the system no longer works.

Try this. Get two pots of water one that holds lets say a half gallon and another that holds one gallon. If the source of the heat is the same which will boil sooner?

Obviously the pot with the smaller quantity of liquid.

Heat spreads. If it spreads into a small reservoir it is going to increase the temp of the liquid or fluid much faster then it would in a larger reservoir.

Another benefit in having a larger reservoir is the larger surface area to dissipate the heat that is generated. Remember the old air cooled four strokes. They all had fins on the cylinder. Why? To provide a larger surface area for heat to dissipate.

All brake systems get hot. It's how that heat is dissipated that matters. This is where larger volumes of fluid and larger surface areas to dissipate that heat help.

my brakes were'nt boiling but getting very hot and fading,i installed a "braking" wave rotor and it helped alot.

Try this. Get two pots of water one that holds lets say a half gallon and another that holds one gallon. If the source of the heat is the same which will boil sooner? Obviously the pot with the smaller quantity of liquid.

This is completely different than what is happening with the brake system. You have a closed system with basically 2 reserviors linked together. One where the fluid is sitting behind the pistons and one at the reservior near the brake pedal. With your analogy, boil one pot of water on the stove with a hose linked to another pot sitting on the counter. The pot on the counter will NEVER boil.

You actually think that much heat travels all the way down the brake line from the pistons to the reservior? I seriously doubt it. The distance is too long and the line too thin. If anything, over that distance the brake line will act as a heat sink, cooling the fluid.

He needs to check the items listed in a post above with regards to dragging pads, warped disk, sand and debris caught between the disk and rotor, etc. The brake fluid should also be changed since it boiled.

I second what vmaxcbr900wr450 said.

I have to wonder how one knows that their brake fluid is boiling or has boiled. It obviously happens as it is mentioned in numerous web sites as issue for some current designed bikes.

I've done some reading on the the boiling point of brake fluid. Performance drops tremendously once moisture is picked up. Brake fluid is very hydroscopic. Down to 284 degrees vs. 401 for dry in a DOT 3 spec. Another thing to look for I suspect is damage to the resivor that would allow moisture into the system. Possibly a small crack that wouldn't leak fluid but

Ok here is my take on it. We know that the systems with smaller fluid capacity have more problems with this. So everytime you heat the fluid it breaks down some, there is some circulation in the system over time. With less fluid the break down is noticable quicker. Used fluid heats quicker and doesn't hold up like new fluid.

So the larger reservior that some are suggesting allows more heat cycles before the fluid becomes unstable.

Check your system. Put new, high quality, fluid in. If over a short amount of time it breaks down again then try the larger reservoir. I ride with some that drag the rear brake all the time, it may just be your style and you have to do a few things, change fluid more often than others, to correct the problem.

Since the fluid boils at the caliper like Sabin said, how does the larger reservior help? You aren't circulating the fluid.

Actually, it does circulate, somewhat.....via convection.

Dirtmizer, make sure you have feeplay in the pedal adjsutment rod connecting the pedal to the master cylinder. If the pedal doesnt allow the piston in the cylinder to return all the way back, it will not "dump" the pressure out of the system via the replenishing port, this will keep some pressure on the brakes, then the extra pressure creates heat in the pads, which is transmitted into the caliper and thus, heating up the fluid. When the fluid gets warmer and the replenish port is closed, the brake fluid pressure will continue to rise and expand, and this just applies the brakes even more resulting in more braking, then more heating, which results in more braking... you get the picture, its a snowball affect. :thumbsup:

How did you find out the fluid is boiling in the caliper ?Yoiu can't see in it.


There is an easy way to confirm this "late braking" debate. Get on you bike and ride. Place hi demand on your brakes for about a half hour. Like maybe several very steep downhill sections with lots of big rocks to avoid. Then stop your bike and place you hand on the rear brake slave unit. Is it warm, hot, or searing? If the caliper has that amount of heat on the external body, what temp would you think the fluid is? Ask yourself if the brakes work the same when cool, or HOT? The answer is cool, as the fluid is more dense.

The brake pads generate heat energy and transfer that energy to the caliper piston via conduction. The piston, whether metal or cermamic (ceramic is the best as it is better at heat insulation, abrasion and chemical erosion resistance) conducts the heat to the fluid along with the caliper body. The whole unit heats up, with the fluid getting thinner and thinner. eventually the brake fails, as it looses it's capacity to pinch the disc sufficiantly. Reduced density of the fluid is the culprit. What do you think all that heat does to the rubber seals in the caliper?

Increasing the fluid volume with the brake system works the same way it does with your rear shock. That is why rear shocks have a resivoir attached to them so when the shock heats up from rebounding, forcing the oil to flow back and forth through the pipit valve, there is more oil volume to sink the heat into. the resivoir body also acts as a heat sink.

OK fellas, enough theory. I went today on a ride to very steep hill. It is 4000ft (1200m) displacement in 4 miles downhill (5.5km). I turned off my engine, so the only stopping power where my brakes. I went down the hill with speed about 15m-30ml/h (25-50km/h). My brakes did perfect!!! I even felt my brakes getting better when brake disks got hot. I use Carbone Lorane Brake pads. Do not try this with stock brake pads, they will not hold! Now the interesting part?on the button of the hill I spit on the calliper and the spit vaporised at the moment. I touched the master cylinder and guess what. IT WAS COLD!!! Not even a little worm! So put whatever cooler to cool the cold master cylinder and hope to fix what do not need to be fixed. Hope this helps?

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