Rear brake soft after bleeding twice

[styran]

The first time I bled the braking system I did it by myself using a vacuum bleeder. The old brake fluid was very dirty. The front brake was super stiff but the rear brake was very soft. A week later, I bled it again but this time using the old fashion method using two people following this guide: https://www.vfrdiscussion.com/index.php?/forums/topic/70842-bleedingfluid-replacement-on-linked-brakes-wabs-on-6th-generation-the-ultimate-guide/

I turned the secondary MC upside down so it was flat with the ground and pushed it several times for the servo PCV and center rear caliper as described in the guide.

I bled until there was clean and airless fluid coming out of all 6 ports.

I skipped the Rear PCV step because my bike does not have a bleed port there for some reason.

 

This took two full hours. The end result was a very stiff front brake (I'm guessing that's because the front lever is not connected to the rear system) and a rear brake that sinks but when you pump it a couple times it starts to get stiff. No where near as stiff as before I opened the brake system but somewhat stiff. Wait like 30 seconds and the pedal sinks again when pressed. I'm guessing there's still air in the rear system but I have no clue where it would be. I bled till there was no air bubbles coming out of anywhere. It is worth noting that the center piston in the left front caliper can be heard when you push the rear brake enough times for it to get somewhat stiff.

 

Any suggestions? I really like the feel of the CBS system compared to traditional systems but maintaining it is proving to be a massive pain in the ass.

Non-ABS 2007 VFR800

 

[RobF]

Interesting. I am having the exact same issue at the moment. I replaced a bunch of brake lines with Galfer stainless-steel lines.

 

I have successfully used @jay-d's awesome guide (which you linked to) in the past, but this time I can't seem to resolve the rear brake. When I began, I had to pump the rear brake four times before pressure developed. Then I realized I was working from memory and not actually following the guide process correctly. Now the rear brake will pressurize after one pump, which is progress but of course not satisfactory. I'm not sure where the remaining air is. Maybe between the pedal and the secondary master cylinder on the left front brake? My next step is probably to pressurize from the pedal, then zip-tie a weight to it so that it remains steadfastly engaged in the hopes that pressuring it overnight will float the elusive bubbles that remain.  I've had luck with this technique in the past, but have already tried it in this case once or twice with mixed results.

[Cogswell]

IIRC in the guide it mentions removing the rear caliper and attaching it to the rotor (through one of the rotor's holes) so that the bleed valve is facing upward.  I find that a tempting step to skip but I always remind myself that it will never get all the air out without doing that.  I'm curious what was meant that the PCV does not have a bleed valve.  Is that to say that there's a plug in place of the bleed valve?  Or it machined without one?  That seems odd - AFAIK all PCV's have a bleed valve ABS or not.  Also, don't forget that the pedal controls more than just the two outer pistons on the rear caliper, but also the center piston on the front calipers so that could be another place where air could be lurking.  With regard to the PCV's, I found that my vacuum bleeder just could not get it done.  I put a post in at the end about getting fluid through, especially in to the SMC on the LF caliper.  The SMC gets filled with fluid by pushing on the rear pedal and is filled from the rear master cylinder, so that also needs to be completely purged for the system to be free from air.  If that post about that I put in isn't the last one in the thread, it's close.  Have a read and see if that could be the problem. 

 

Edit:

 

You can check to see if you have air in the SMC.  Get a helper - with the bike on the C-stand, grasp the SMC and compress it as much as possible by hand.  While holding it as firmly as possible,  have the helper attempt to rotate the rear wheel.  There should be noticeable drag, but it probably won't be enough to lock the wheel.  If it turns freely, there is air in the SMC.  Since the SMC is filled by action from the pedal, there is a good chance that is the source of the issue.  Go through the process I outlined to fill the SMC and get the air out through the shifter side PCV then see what you've got.  Don't forget to check the rear MC frequently, it will go through a lot to get the SMC full and free of air. 

 

 

[styran]

47 minutes ago, Cogswell said:

IIRC in the guide it mentions removing the rear caliper and attaching it to the rotor (through one of the rotor's holes) so that the bleed valve is facing upward.  I find that a tempting step to skip but I always remind myself that it will never get all the air out without doing that.  I'm curious what was meant that the PCV does not have a bleed valve.  Is that to say that there's a plug in place of the bleed valve?  Or it machined without one?  That seems odd - AFAIK all PCV's have a bleed valve ABS or not.  Also, don't forget that the pedal controls more than just the two outer pistons on the rear caliper, but also the center piston on the front calipers so that could be another place where air could be lurking.  With regard to the PCV's, I found that my vacuum bleeder just could not get it done.  I put a post in at the end about getting fluid through, especially in to the SMC on the LF caliper.  The SMC gets filled with fluid by pushing on the rear pedal and is filled from the rear master cylinder, so that also needs to be completely purged for the system to be free from air.  If that post about that I put in isn't the last one in the thread, it's close.  Have a read and see if that could be the problem. 

 

Edit:

 

You can check to see if you have air in the SMC.  Get a helper - with the bike on the C-stand, grasp the SMC and compress it as much as possible by hand.  While holding it as firmly as possible,  have the helper attempt to rotate the rear wheel.  There should be noticeable drag, but it probably won't be enough to lock the wheel.  If it turns freely, there is air in the SMC.  Since the SMC is filled by action from the pedal, there is a good chance that is the source of the issue.  Go through the process I outlined to fill the SMC and get the air out through the shifter side PCV then see what you've got.  Don't forget to check the rear MC frequently, it will go through a lot to get the SMC full and free of air. 

 

 

I had the rear caliper mounted with the ports facing up when I bled the system both times. On the right side of the bike where the rear pcv bleed port is supposed to be there is pretty much nothing there. Just a couple brake lines and empty space. I'm probably gonna end up rebleeding the servo PCV (battery side) and see if helps the issue. I will be surprised if any air comes out as it took 20 minutes just to get airless fluid out of that line.

[Kbear]

You can view this video if you like. I don't get into all this guys videos but maybe you can glean some information from him that might help?

 

 

[Cogswell]

In the video, at the 24:30 mark he goes in to what I mentioned about the SMC.  It is filled with fluid from the rear MC using the rear brake pedal.  If there is any air in it, a vacuum bleeder will never move fluid through there if you bleed it for a year.  It must be done by using the force of the rear MC to force fluid through.  As that happens, keep a hand at all times on the SMC - you can feel it being pushed out by hydraulic pressure.  Then crack the PCV bleeder open and push the SMC closed, then close the PCV bleeder and again use the brake pedal to force more fluid though. Never allow the SMC to extend on its own - only by pressure from the pedal.   Rinse and repeat.  On that part of the system, a vaccum bleeder will pull a small amount of fluid through if the system is already purged of air - so changing some of the brake fluid in the SMC that way is possible (though will take an eternity - the best way of changing it is as he describes).  If any air has gotten in to the SMC, it just ain't happenin' with a vacuum bleeder - what he demonstrates and what's in the thread is the only way.  This could very well be the source of the spongy rear brake pedal.  Hydraulic pressure will take the path of least resistance and if the SMC has air the pedal will compress it 1st.   Plus, you won't be getting full braking as the SMC will be doing nothing.  Checking to see if the SMC activates the rear caliper center piston is a good way to see if this is the issue. 

[styran]

45 minutes ago, Cogswell said:

In the video, at the 24:30 mark he goes in to what I mentioned about the SMC.  It is filled with fluid from the rear MC using the rear brake pedal.  If there is any air in it, a vacuum bleeder will never move fluid through there if you bleed it for a year.  It must be done by using the force of the rear MC to force fluid through.  As that happens, keep a hand at all times on the SMC - you can feel it being pushed out by hydraulic pressure.  Then crack the PCV bleeder open and push the SMC closed, then close the SMC bleeder and again use the brake pedal to force more fluid though. Never allow the SMC to extend on its own - only by pressure from the pedal.   Rinse and repeat.  On that part of the system, a vaccum bleeder will pull a small amount of fluid through if the system is already purged of air - so changing some of the brake fluid in the SMC that way is possible (though will take an eternity - the best way of changing it is as he describes).  If any air has gotten in to the SMC, it just ain't happenin' with a vacuum bleeder - what he demonstrates and what's in the thread is the only way.  This could very well be the source of the spongy rear brake pedal.  Hydraulic pressure will take the path of least resistance and if the SMC has air the pedal will compress it 1st.   Plus, you won't be getting full braking as the SMC will be doing nothing.  Checking to see if the SMC activates the rear caliper center piston is a good way to see if this is the issue. 

Should I be holding the brake pedal down while pushing in the SMC? When I bled it I just let go of the pedal.

In my procedure I:

1. pumped rear pedal until SMC is fully forced out

2. let go of brake pedal

3. start pushing SMC

4. open PCV valve

5. close 

6. repeat

If I'm supposed to hold it down while pushing SMC then that would give me a clue on what to do next.

 

[Cogswell]

Once the pedal has pushed the SMC out, it can be released - it will then draw fluid from the rear MC.  The key thing is to not allow the SMC to move in or out on its own - only out by the pedal and in by compressing it by hand.  The SMC is a small bore and moves little fluid. It takes a significant number of cycles for the pedal / SMC to move fluid from the rear MC out the PCV. If the SMC is allowed to extend while the PCV bleeder is open, air will be drawn in , so close the bleeder while keeping the SMC compressed.   Then extend the SMC with the pedal,  release the pedal, begin compressing the SMC then open the PCV bleeder.  Repeat the cycle. The SMC should do nothing to draw fluid - only move it along toward the PCV and should never move on its own during the process. 

 

If you've done it that way and compressing the SMC by hand results in some rear brake drag then the SMC should be good - the air giving you trouble is somewhere else in the system, possibly the front calipers' center piston.