Topic: CB750 master cylinder rebuild and bleeding

[chiefum]

My front brake was dragging, so I decided to replace the pads and rebuild the master cylinder on my k1. I used a honda rebuild kit for the master cylinder and I believe everything went together correctly.  When I try to bleed the brake, I can slowly build some pressure,  enough the stop the wheel when the bike is lifted. But I can not seem to get a solid lever. I am able to pull the lever all the way back to the grip. I've never bled a motorcycle before, but I've done cars plenty of times.  What am I doing wrong? Does it seem more like air in the line, or does it sound like I messed up rebuilding the master cylinder? Any help would be appreciated.
Thanks!

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[jaytee-nz]

If you can pull the lever back all the way, you still have air in the lines so more bleeding required. One thing to note is that when you are pulling the lever during bleeding (assuming you are doing it the traditional way through the bleed valve on the caliper) do not pull the lever all the way back to the bars. If you pull it too far you will end up with more air in your lines due to the design of the M/C. About two thirds pull is all you need.

[scottly]

You can also try "reverse" bleeding, by pressing the caliper piston back in, which forces air uphill and back into the MC through the tiny hole. The tiny hole tends to trap air bubbles, as they are too big to fit through the hole, and traditional bleeding doesn't move enough fluid to push the air down and into the caliper; it rises back up in the line between strokes. Forcing the piston back in creates enough pressure to force the air though the relief hole.

[chiefum]

If you can pull the lever back all the way, you still have air in the lines so more bleeding required. One thing to note is that when you are pulling the lever during bleeding (assuming you are doing it the traditional way through the bleed valve on the caliper) do not pull the lever all the way back to the bars. If you pull it too far you will end up with more air in your lines due to the design of the M/C. About two thirds pull is all you need.

Thank you! I've been working on this off and on for two days  now, I'll probably give it another try tomorrow.

[chiefum]

You can also try "reverse" bleeding, by pressing the caliper piston back in, which forces air uphill and back into the MC through the tiny hole. The tiny hole tends to trap air bubbles, as they are too big to fit through the hole, and traditional bleeding doesn't move enough fluid to push the air down and into the caliper; it rises back up in the line between strokes. Forcing the piston back in creates enough pressure to force the air though the relief hole.

I'm not sure if I did that correctly or not. What I ended up doing was to pump the piston out some, and then I pushed the piston back in as far as I could.

[chiefum]

I did pick up a mighty vac,  so I might give that a try tomorrow and see if that does any better.

[stocky]

I just went through all of this recently after doing the dual disc conversion, replaced all the lines with SS, rebuilt both calipers, new pads, rebuilt GL1000 MC and it was a nightmare trying to get all the air out. It was probably worse since it was starting from a new dry system and I never really worked on anything this old.

It was frustrating to say the least since I tried everything including the mityvac and it didn't help much. After trying a bunch of methods and going through almost 3 bottles, I found reverse bleeding from the calipers was most effective. Lot of videos on YT on how to do this but basically just get a big syringe and fill it up with brake fluid, pump it up through the bleeder valve till the fluid fills the reservoir, once it fills, close the bleeder valve and empty the fluid with a baster. Repeat this process several times and pray you get good brake pressure and feedback. If this didn't work for me, I probably would have ditched all of it for something new. 

Edit to add: Also tying the brake lever down overnight seems to help a bit as well, so you can give that a shot, good luck!

[jonda500]

I have found it helpful to bleed the system further up closer to the master cylinder using my finger as the bleed valve - can be messy but at least you find out if the master is pumping fluid down properly?
John

[chiefum]

I just went through all of this recently after doing the dual disc conversion, replaced all the lines with SS, rebuilt both calipers, new pads, rebuilt GL1000 MC and it was a nightmare trying to get all the air out. It was probably worse since it was starting from a new dry system and I never really worked on anything this old.

It was frustrating to say the least since I tried everything including the mityvac and it didn't help much. After trying a bunch of methods and going through almost 3 bottles, I found reverse bleeding from the calipers was most effective. Lot of videos on YT on how to do this but basically just get a big syringe and fill it up with brake fluid, pump it up through the bleeder valve till the fluid fills the reservoir, once it fills, close the bleeder valve and empty the fluid with a baster. Repeat this process several times and pray you get good brake pressure and feedback. If this didn't work for me, I probably would have ditched all of it for something new. 

Edit to add: Also tying the brake lever down overnight seems to help a bit as well, so you can give that a shot, good luck!

Thanks!! Do you have any info on the syringe you used? Or where you purchased? I'm willing to try anything,  I just want to get it back out on the road again.

[chiefum]

I have found it helpful to bleed the system further up closer to the master cylinder using my finger as the bleed valve - can be messy but at least you find out if the master is pumping fluid down properly?
John

Thank you! I will possibly try that.

[Medyo Bastos]

I just went through all of this recently after doing the dual disc conversion, replaced all the lines with SS, rebuilt both calipers, new pads, rebuilt GL1000 MC and it was a nightmare trying to get all the air out. It was probably worse since it was starting from a new dry system and I never really worked on anything this old.

It was frustrating to say the least since I tried everything including the mityvac and it didn't help much. After trying a bunch of methods and going through almost 3 bottles, I found reverse bleeding from the calipers was most effective. Lot of videos on YT on how to do this but basically just get a big syringe and fill it up with brake fluid, pump it up through the bleeder valve till the fluid fills the reservoir, once it fills, close the bleeder valve and empty the fluid with a baster. Repeat this process several times and pray you get good brake pressure and feedback. If this didn't work for me, I probably would have ditched all of it for something new. 

Edit to add: Also tying the brake lever down overnight seems to help a bit as well, so you can give that a shot, good luck!

Thanks!! Do you have any info on the syringe you used? Or where you purchased? I'm willing to try anything,  I just want to get it back out on the road again.

A turkey basting syringe.  Be easy on how hard you squeeze. It doesn’t take much to get brake fluid up that skinny line!


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[chiefum]

I just went through all of this recently after doing the dual disc conversion, replaced all the lines with SS, rebuilt both calipers, new pads, rebuilt GL1000 MC and it was a nightmare trying to get all the air out. It was probably worse since it was starting from a new dry system and I never really worked on anything this old.

It was frustrating to say the least since I tried everything including the mityvac and it didn't help much. After trying a bunch of methods and going through almost 3 bottles, I found reverse bleeding from the calipers was most effective. Lot of videos on YT on how to do this but basically just get a big syringe and fill it up with brake fluid, pump it up through the bleeder valve till the fluid fills the reservoir, once it fills, close the bleeder valve and empty the fluid with a baster. Repeat this process several times and pray you get good brake pressure and feedback. If this didn't work for me, I probably would have ditched all of it for something new. 

Edit to add: Also tying the brake lever down overnight seems to help a bit as well, so you can give that a shot, good luck!

Thanks!! Do you have any info on the syringe you used? Or where you purchased? I'm willing to try anything,  I just want to get it back out on the road again.

A turkey basting syringe.  Be easy on how hard you squeeze. It doesn’t take much to get brake fluid up that skinny line!


Sent from my iPhone using Tapatalk

Awesome,  thank you!

[stocky]

Thanks!! Do you have any info on the syringe you used? Or where you purchased? I'm willing to try anything,  I just want to get it back out on the road again.

Used a pump syringe thing I got off amazon years ago for other automotive stuff. And used the baster to extract the fluid from the reservoir. Attached a few pictures below.

 

A turkey basting syringe.  Be easy on how hard you squeeze. It doesn’t take much to get brake fluid up that skinny line!

Mine actually took a bit of effort, wasn't hard but definitely took a strong firm push to get fluid to start flowing, though maybe I didn't crack my bleeder valve open enough, not sure what other people experiences are since it was my first time reverse bleeding and my setup is no longer stock..ish

[chiefum]

Thanks!! Do you have any info on the syringe you used? Or where you purchased? I'm willing to try anything,  I just want to get it back out on the road again.

Used a pump syringe thing I got off amazon years ago for other automotive stuff. And used the baster to extract the fluid from the reservoir. Attached a few pictures below.

 

A turkey basting syringe.  Be easy on how hard you squeeze. It doesn’t take much to get brake fluid up that skinny line!

Mine actually took a bit of effort, wasn't hard but definitely took a strong firm push to get fluid to start flowing, not sure what other people experiences are since it was my first time reverse bleeding and my setup is no longer stock..ish

Thank you!!

[seanbarney41]

I have found it helpful to bleed the system further up closer to the master cylinder using my finger as the bleed valve - can be messy but at least you find out if the master is pumping fluid down properly?
John

Thank you! I will possibly try that.

a couple things to try...completely emptied mc often has lots of air trapped that won't get out.  Take your top brake line and loop it back into the open reservoir of the mc, submerging it in the brake fluid.  Then pump the lever gently until no more air comes out.

Also, as said above by jonda, I like to get a big old towel or rag and hold it under the mc banjo bolt, crack the banjo open, and bleed here just as you would with the bleed screw.

It's screwy!  I have never had much success bleeding only with the screw on the caliper.  Sometimes all it takes is to get the system bled as good as possible, then if you know you have a safe test area, just go ride it and vibrate the air out.  Slam on the brakes a couple times, pump like mad.  I have had bikes where that is all that works.

[chiefum]

I have found it helpful to bleed the system further up closer to the master cylinder using my finger as the bleed valve - can be messy but at least you find out if the master is pumping fluid down properly?
John

Thank you! I will possibly try that.

a couple things to try...completely emptied mc often has lots of air trapped that won't get out.  Take your top brake line and loop it back into the open reservoir of the mc, submerging it in the brake fluid.  Then pump the lever gently until no more air comes out.

Also, as said above by jonda, I like to get a big old towel or rag and hold it under the mc banjo bolt, crack the banjo open, and bleed here just as you would with the bleed screw.

It's screwy!  I have never had much success bleeding only with the screw on the caliper.  Sometimes all it takes is to get the system bled as good as possible, then if you know you have a safe test area, just go ride it and vibrate the air out.  Slam on the brakes a couple times, pump like mad.  I have had bikes where that is all that works.

Thank you!!! I'm hoping to have some time tomorrow to try some different things. I'm thankful for all the great suggestions.

[chiefum]

After trying everything to bleed the line, I took the master cylinder apart and found that it had some crusty gunk in it, not a lot, but I missed it on my first time around. After not being able to find a small enough hone, I read through the forum and found Bankerdanny linked an article where they used fine emery cloth glued to fuel hose. So I gave that a try, then reverse bled the system. Success!! I have a front brake again. Thanks to everyone that contributed!

[chiefum]

Reverse bleeding this time took about five minutes. This is the kit that I ordered.

https://www.brakecrafters.com/products/reverse-bleeding-kit

[Bankerdanny]

After trying everything to bleed the line, I took the master cylinder apart and found that it had some crusty gunk in it, not a lot, but I missed it on my first time around. After not being able to find a small enough hone, I read through the forum and found Bankerdanny linked an article where they used fine emery cloth glued to fuel hose. So I gave that a try, then reverse bled the system. Success!! I have a front brake again. Thanks to everyone that contributed!

That was from Motorcycle Classics CB500 restoration.

[chiefum]

After trying everything to bleed the line, I took the master cylinder apart and found that it had some crusty gunk in it, not a lot, but I missed it on my first time around. After not being able to find a small enough hone, I read through the forum and found Bankerdanny linked an article where they used fine emery cloth glued to fuel hose. So I gave that a try, then reverse bled the system. Success!! I have a front brake again. Thanks to everyone that contributed!

That was from Motorcycle Classics CB500 restoration.

Yes! Thank you for posting the link! It worked well.