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8Rider6

No rear brake pressure

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I did a full brake fluid flush last year, front and rears. 

About 2-3 months ago, I had no rear brakes/pressure on my way to work one day. That evening, I flushed the rear brakes and it was good for another 2-3 months until today. I checked them before I rode since I hadn't ridden that bike for 3-4 weeks and wanted to know if I had my rear brake before setting off. 

Any ideas what's going on? If it works fine after new fluid, I would think the master cylinder and brake line are OK? It should be good for several years without replacement though, not a few weeks or months. 

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Air bubbles or a fluid leak?

 

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Unknown currently. When it happened before, no leak. There may have been air when I bled them 2-3 months ago, but I kept refilling and bleeding the line until I saw no bubbles (as was done around a year ago). 

I am wondering if air is somehow being introduced after I flush the line/replace the fluid. 

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Find something to press down on the lever and leave it overnight. Is there brake pressure the next day? Or is it all the way to the "floor"? When I do this after about 1-2 pumps the lever is stiff again and good to go.

Example: When I do my front brakes I zip tie my brake lever overnight with the brake on to make sure I don't have a slow leak anywhere. Zip tie it almost as far as I can pull the lever.

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49 minutes ago, TimTheAzn said:

Find something to press down on the lever and leave it overnight. Is there brake pressure the next day? Or is it all the way to the "floor"? When I do this after about 1-2 pumps the lever is stiff again and good to go.

If that works, what would that mean? I did try to pump the lever about 8-10 times and no more pressure built up after trying. 

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3 minutes ago, 8Rider6 said:

If that works, what would that mean? I did try to pump the lever about 8-10 times and no more pressure built up after trying. 

He wants you to bleed it and fix it first, then do his test.

If it goes to the floor it most probably means the master cylinder is bad, it is bypassing. And that would be the source of the air that is slowly getting into the system. It is bypassing and letting in a little air each time you use it...takes months to build up enough air to fail.

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12 minutes ago, 8Rider6 said:

If that works, what would that mean? I did try to pump the lever about 8-10 times and no more pressure built up after trying. 

Doing that now may push some fluid out instead of letting air in, letting you know where the leak is. Whether its the brake line itself, or bleeder screw. If the only thing you did was flush the rear brake, what fittings did you all touch? I doubt you undid the brake line itself, but I bet you touched that bleeder screw(s). Im assuming your rear master has a bleeder screw as well as the caliper itself?

Edit: You flushed because you found you had no brake pressure. My suggestion is still legit to see where it may be leaking.

7 minutes ago, Tonik said:

He wants you to bleed it and fix it first, then do his test.

If it goes to the floor it most probably means the master cylinder is bad, it is bypassing. And that would be the source of the air that is slowly getting into the system. It is bypassing and letting in a little air each time you use it...takes months to build up enough air to fail.

Can be done before and after. Before in the hopes of pushing some fluid out to show the user where the leak may be. After to confirm no more leaks.

Edited by TimTheAzn

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Gotcha, I understand the purpose of that test now. I'll probably try it as is tonight since I don't have time to bleed it tonight, bleed it tomorrow night and try it again and see what comes of it. 

Correct, I didn't disassemble the line, just touched the bleeder screw/valve on the caliper and the cap and diaphragm for the master cylinder. I don't *think* there's bleeder screw on the master cylinder itself. The bike is a 2002 Honda 919 for reference. 

Diagram for the master cylinder is here. Unless it's part #21, I don't think there's a bleeder screw. 

https://www.bikebandit.com/oem-parts/2007-honda-919-cb900f/o/m151030#sch419715

Edited by 8Rider6

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37 minutes ago, 8Rider6 said:

Gotcha, I understand the purpose of that test now. I'll probably try it as is tonight since I don't have time to bleed it tonight, bleed it tomorrow night and try it again and see what comes of it. 

Correct, I didn't disassemble the line, just touched the bleeder screw/valve on the caliper and the cap and diaphragm for the master cylinder. I don't *think* there's bleeder screw on the master cylinder itself. The bike is a 2002 Honda 919 for reference. 

Diagram for the master cylinder is here. Unless it's part #21, I don't think there's a bleeder screw. 

https://www.bikebandit.com/oem-parts/2007-honda-919-cb900f/o/m151030#sch419715

Yup, pump that rear brake up so its got good pressure then find something to hold it and come back the next day, see if any fluid came out.

I would use brake clean to clean the caliper and master so any new fluid that may or may not appear overnight is obvious.

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Nope, not before or after flushing the system again. Pressure is back for now anyway, but who knows how long it'll last. I may buy a master cylinder rebuild kit and try my hand at it once the bike is done for the season. 

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