Soft rear brake

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C Graham

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My rear brake became soft over the winter. I could push it all the way down while riding and not engage the abs or front piston. I bled the rear and it felt normal for about 400 miles then became soft again. I have bled the rear two more times but am not getting results.

The rear pads still look nice and fat so I don't think it's that.

It's an 08 and I'm using dot 4 if that matters.

Any suggestions?

 
My rear brake became soft over the winter. I could push it all the way down while riding and not engage the abs or front piston. I bled the rear and it felt normal for about 400 miles then became soft again. I have bled the rear two more times but am not getting results.

The rear pads still look nice and fat so I don't think it's that.

It's an 08 and I'm using dot 4 if that matters.

Any suggestions?
Did you bleed the front linked brake? You need to do them both. I'm betting you still have an air bubble in there somewhere.

 
Yes, the rear pedal operates the front brake's lower piston pair on the right side. That should be bled, too.
Yup. +1 an all that...

If my bike sits for a month, it has the same problem.

I've looked for leaks or any other reason.

?

Weird...

 
Complete brake fluid flush might help, as the fluid absorbs water over time, and water does not behave well under heat and pressure.

If a bleed screw doesn't seal, the system might be pulling air into the cylinder behind the piston.

 
Complete brake fluid flush might help, as the fluid absorbs water over time, and water does not behave well under heat and pressure.

If a bleed screw doesn't seal, the system might be pulling air into the cylinder behind the piston.
Flesh frush every year on it's birthday...

Prolly that chinese copy brake fluid we get.....

Oh well...

When it don't work, fix it....

 
I have the same problem. Flush didn't fix it and the mech spent a lot of time looking for any air hidden in the lines. Also, the rear brake won't pump solid like it normally would with air in the line. I had it looked at again along with my valve clearance service and a new rear master cylinder is waiting at the dealer to be installed with the spider recall. Mech says that he's replaced a half dozen or so for same problem. Hope to have this all taken care of before NERDS.

 
I have the same problem. Flush didn't fix it and the mech spent a lot of time looking for any air hidden in the lines. Also, the rear brake won't pump solid like it normally would with air in the line. I had it looked at again along with my valve clearance service and a new rear master cylinder is waiting at the dealer to be installed with the spider recall. Mech says that he's replaced a half dozen or so for same problem. Hope to have this all taken care of before NERDS.
Interesting. Be sure to let us know if the new master cylinder fixes your issue.

My old g/f's 2009 BMW F650GS suffers from the same affliction. We have had the rear brake flushed/bled on a regular basis. It doesn't take long before it gets all soft again. Next time, I'll get on the dealer about a new master cylinder.

 
I have the same problem. Flush didn't fix it and the mech spent a lot of time looking for any air hidden in the lines. Also, the rear brake won't pump solid like it normally would with air in the line. I had it looked at again along with my valve clearance service and a new rear master cylinder is waiting at the dealer to be installed with the spider recall. Mech says that he's replaced a half dozen or so for same problem. Hope to have this all taken care of before NERDS.
Interesting. Be sure to let us know if the new master cylinder fixes your issue.

My old g/f's 2009 BMW F650GS suffers from the same affliction. We have had the rear brake flushed/bled on a regular basis. It doesn't take long before it gets all soft again. Next time, I'll get on the dealer about a new master cylinder.
If the brake system isn't leaking, if the integrated brake system and ABS metering block have been correctly bled and the brake pedal isn't firm and can't be pumped up then the only cause can be a master cylinder that is not sealing correctly. A new Master Cylinder may fix the problem, the question now becomes why a low mileage MS has failed. Recall, seals or plunger bore tolerances can be the only cause. (Unless it is something else not revealed or not recognized in the user failure analysis, as opposed to a technician's evaluation.)

 
I ran a full bottle of brake fluid through the rear only and that seems to have fixed the problem. Is very firm. Will see if it lasts.

 
I ran a full bottle of brake fluid through the rear only and that seems to have fixed the problem. Is very firm. Will see if it lasts.
But did you bleed the linked-front caliper? Think you're gonna be chasing undomesticated fowl unless you do the entire job.

 
Don't have a problem with the front but am going to bleed them anyway.

For the linked system (right side caliper) do I pump rear brake then bleed or front brake then bleed???

On a positive note, I just got my recall notice from yamaha. Called my dealer to schedule the repair and was told if I bring the new plugs I was planning on installing he would just throw them in for me while he had the tank off.

BONUS!!!!

 
Don't have a problem with the front but am going to bleed them anyway.

For the linked system (right side caliper) do I pump rear brake then bleed or front brake then bleed???

On a positive note, I just got my recall notice from yamaha. Called my dealer to schedule the repair and was told if I bring the new plugs I was planning on installing he would just throw them in for me while he had the tank off.

BONUS!!!!
The right caliper has two bleed screws. The upper one is bled with the front brake lever and the lower one is bled using the rear brake pedal. This piston is ONLY actuated with the rear brake pedal while the front brake lever actuates the two left pistons and the upper right only.

Ross

 
And we are assuming you know how to bleed brake lines... Press lever, hold it in place, unscrew bleeder, rescrew bleeder, let go of lever, making sure your master is always full (or plenty full) of Dot oil. Best is to have a buddy help you out. God help you if you let go of that lever even a bit with the bleeder still open.

 
And we are assuming you know how to bleed brake lines... Press lever, hold it in place, unscrew bleeder, rescrew bleeder, let go of lever, making sure your master is always full (or plenty full) of Dot oil. Best is to have a buddy help you out. God help you if you let go of that lever even a bit with the bleeder still open.
Or easier yet, install some Speedbleeders and it's a one man job!

Crap...something else I need to buy... ;)

 
After long-term weak rear brake...a tech suggested elevating the caliper when doing the deed.

Yesterday I swapped tires, and with the rear wheel removed, we raised the caliper, and banged on it w/ a rubber mallet. I got 2 HUGE (1 inch+) bubbles out of the line. Now the rear is hard as can be (hope it holds) and the fluid is clean.

Keep in mind that you need a spare rotor to do this elevated w/o the tire. Also I have non-ABS and speedbleeders which make things easier....but still....air rises to the top! ;)

 
I have the same problem. Flush didn't fix it and the mech spent a lot of time looking for any air hidden in the lines. Also, the rear brake won't pump solid like it normally would with air in the line. I had it looked at again along with my valve clearance service and a new rear master cylinder is waiting at the dealer to be installed with the spider recall. Mech says that he's replaced a half dozen or so for same problem. Hope to have this all taken care of before NERDS.
Interesting. Be sure to let us know if the new master cylinder fixes your issue.

....
I had the rear brake master cylinder replaced yesterday along with the ground spider recall. Brake is as solid as new and I have proper rear braking again. May need to adjust it down a bit just to make certain my foot doesn't ride on it unintentionally. Mech adjusted it lower for me but I think it should be a bit lower yet. He pinched off the line from the reservoir, removed the bolts holding the bracket for the pegs and then removed the banjo bolt, replaced the master cylinder and then bled the lines. Didn't take him very long and when he was done, he went and washed off the bike to make certain that there wasn't any brake fluid sitting where it wasn't wanted. He even did some polishing where he may have left prints. Thorough job. I was worried a bit when I arrived at the dealer and found out that my order for the banjo crush washers was screwed up by the parts counter but mech had some in his private stock and saved me another trip. The mech is the primary reason I get the service done there that I don't want to do personally.

 
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