Rear brake failure?

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iwantone

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03 bumble. I replaced my rear pads about 1000 or so miles ago. I was riding down a mountain road the other day and my rear brake lever started getting a little spongey. Didn't think anything of it. I get a few miles farther and I lose all pressure in my rear brakes. I pushed on the lever as far down as it would let me, I then started hearing some grinding. I stop at the end of the road, get off my bike to check if any fluid is leaking or if the reservoir is low. Both seem to be fine. I also check my caliper and out also looks fine. I figure I can get home with just the front brakes if I drive slow enough. Out of habit, I press on the reat lever and all pressure is restored. I even tested the stopping power and it all seems to be back to normal.

My question is, what would cause temporary loss of my rear brakes?

I typed this on my phone, so please excuse the typos :)

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Were you riding the rear brake while riding down the hill? It's possible that you boiled the fluid, I suppose. Not sure how that would cause the grinding noise, though.

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Old fluid can have a high moisture content and cause vapor lock when the water boils and creates steam(pressure) in the braking system.
 
See I thought it was heat too, but I just got doing a 150 mile ride with no issues. Plus, before this happened my bike sat parked for a good 30 minutes.

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IBcivic said:
Old fluid can have a high moisture content and cause vapor lock when the water boils and creates steam(pressure) in the braking system.
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Yeah, it has 25k on it. Not really sure if the fluid has ever been replaced. What the best way to bleed after replacing the fluid?

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iwantone said:
What are the chances of it being the rear master cylinder

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Usually there is leakage around the rubber dust collar if the cylinder starts to fail. It is possible so I would check for leaks, and do that at the brake end as well. Old fluid or overheated fluid will cause brake fade as well, like others have said.
 
iwantone said:
Would it be wise to spray the caliper off with brake cleaner then check or no?

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If you want to tell if it's leaking don't clean it off. just grab a white rag and pull the rubber boot back and wipe around the bottom around the steel shaft. see if you find any brake fluid.
 
iwantone said:
Yeah, it has 25k on it. Not really sure if the fluid has ever been replaced. What the best way to bleed after replacing the fluid?

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If you have a bit of patience you can normally just open the bleeder screw and let it drain out keep the reservoir open and just refill as it gets low. When new brake fluid is clean you got it bled out.

You can also buy a vacuum kit and change fluid that way.

Or you can just drain it all out... Refill and pump and hold and release the pressure in the screw. Repeat till no air is left.

You need to change your fluid at minimum every couple years. If you look at your master cylinder you will see a very small hole to allow for pressure variance in the the system. That also lets moisture in over time.
 
This is how I do it:

I use a small hose with a one way valve in it. I put it on the bleeder nipple, open it up and start pumping the old fluid through. Once it's coming out clear, I close the bleeder, pump and hold the brakes and open the bleeder. Once I no longer see air bubbles, I'm done. Doesn't take long
 
Dustin said:
This is how I do it:

I use a small hose with a one way valve in it. I put it on the bleeder nipple, open it up and start pumping the old fluid through. Once it's coming out clear, I close the bleeder, pump and hold the brakes and open the bleeder. Once I no longer see air bubbles, I'm done. Doesn't take long
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Ha I totally forgot about that as I was typing. Lisle makes a kit for that. It's pretty cheap, it worked wonders on my dodge. You just have to keep the cup below bleeder crack the screw and gently press on the brakes. It will push fluid out and a check valve in the cup keeps air from going back in.

Dustin said:
I thought brake cleaner was safe for rubber, ie. won't dry it out because it's designed to evaporate quickly. :dontknow:
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It is. Just spraying rubber isn't gonna hurt it. Soaking it long term will.
 
Same thing happened to me last year. I have had it happen on an off road bike as well. Order yourself a master cylinder rebuild kit and install it and you should be good to go.
 
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