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Discussion Starter #1
Hi Folks,


Couple of quick and possibly dumb questions relating to my 2018 959 rear brake...


1. My rear brake is spongy until the bike warms up and then it feels as firm as it should do. I'm thinking that there's air in the line somewhere and the air is expanding from the engine heat and causing the improved pedal feel? Am I correct or talking nonsense?



2. From searching the interwebs I know that to bleed the line I need to get the caliper higher than the master cylinder to allow the air to rise to the bleed valve. Problem is that the brake line and ABS cable are fixed to the inside of the swingarm by a clip which seems to be screwed into or bolted to the swingarm. Without removing the clip and freeing the brake line I can't get the caliper high enough, so it looks like the rear wheel needs to come off to access the clip. I can do that, but before I do,I wanted to make sure I wasn't missing some obvious way of reorienting the caliper relative to the master cylinder.


Thanks,


Davy
 

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I have bled the rear brake 3 times since I purchased the bike, and never removed the caliper.

I just did the pump and twist/release the bleeder screw method until I ran several reservoirs worth of fluid through the lines. No spongy feeling at all following that method.

Try that simple method first and see if the pedal feels as it should. Most likely that will be good enough.

The video below shows how to do it using a brake vac if you have one.

 

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Discussion Starter #3
I have bled the rear brake 3 times since I purchased the bike, and never removed the caliper.

I just did the pump and twist/release the bleeder screw method until I ran several reservoirs worth of fluid through the lines. No spongy feeling at all following that method.

Try that simple method first and see if the pedal feels as it should. Most likely that will be good enough.

The video below shows how to do it using a brake vac if you have one.

https://www.youtube.com/watch?v=0Nm7wxd8KoI

Thanks for the reply. I've bled them the normal way and it made no difference to the spongy feel. Everything I've read says that the bleed valve needs to be higher than the caliper otherwise the air will stay trapped in the line or master cylinder.


Davy
 

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You could reverse bleed them by pushing fluid from the bleed nipple through the caliper and up through the master cyl. I'm pretty sure you can do with a vacuum bleeder.
 

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1. My rear brake is spongy until the bike warms up and then it feels as firm as it should do. I'm thinking that there's air in the line somewhere and the air is expanding from the engine heat and causing the improved pedal feel? Am I correct or talking nonsense?
Even though air has a larger thermal volumetric expansion coefficient than brake fluid, the amount of air is negligible in relation to the amount of fluid in the system. They both expand with temperature. Ultimately, temperature doesn't really affect sponginess because of the variable-volume nature of the reservoir (it relieves pressure to reservoir, always constant volume on the working side), however the total amount of air trapped in the working side of the hydraulic system does.

More importantly when talking about temperature and brake fluid is the change in brake fluid viscosity. You get improved brake feel with temperature because the viscosity in the fluid drops with higher temperature. Less viscous fluid = less dynamic losses against the brake lines and thus better feel.


2. From searching the interwebs I know that to bleed the line I need to get the caliper higher than the master cylinder to allow the air to rise to the bleed valve. Problem is that the brake line and ABS cable are fixed to the inside of the swingarm by a clip which seems to be screwed into or bolted to the swingarm. Without removing the clip and freeing the brake line I can't get the caliper high enough, so it looks like the rear wheel needs to come off to access the clip. I can do that, but before I do,I wanted to make sure I wasn't missing some obvious way of reorienting the caliper relative to the master cylinder.
I've accidentally let air into the rear brake before. Bled it normally (without raising it) to a hard pedal without too much of a problem. Eventually I raised the rear caliper high the one time I bled it and it made a small, but noticeable difference. I just happened to have the rear wheel off, so I thought I might as well finally bleed it with the nipple facing upward so bubbles would go to the high point. I don't remember if I made it above the master, but all I did was point the nipple up and hang it for ~30 minutes from the tail.


It's worth doing if you have your rear wheel off, but don't worry about it otherwise. Worry more about the front brake and make sure it is 100%.
 
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I've always just used the old-fashioned bleeder screw / lever pump method. Aside from maybe one occasion where I actually had to reverse bleed a clutch line, it has always done the trick for me. Keep it simple!!
 
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