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Discussion Starter #1
Bike is a 2008 ABS with a serious braking system problem. Initial pressure on the rear brake pedal will drive pedal all the way down to it's stop. Pumping the pedal will bring the pedal back to a firm position but within a minute or two, the pedal will again sink all the way down under foot pressure. I've bled the brakes controlled by the rear master cylinder 3 times and each time showed no air bubbles whatsoever. There is no sign of fluid loss. Bleeds were done in the sequence specified in the shop manual. There sure seems to be air in the system but I'll be darned if I can find it.
As I understand it, there are additional bleed points and I have seen reference to the "Rocky Bleed" as a possible solution. Any help with this problem would be most welcome.
 

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A bit of clarification on the symptoms you're observing:

1.) Immediately after bleeding the brakes, as you have done; is the Brake Pedal firm when you first apply pressure with your foot/hand, just (as in immediately) after you have closed the Bleed Screw(s)?

2.) Without taking any pressure off the Brake Pedal as described in Step 1 above; if you continue to hold a fairly constant pressure on the Brake Pedal for one minute (or even a bit more); does the Brake Pedal hold position, or does it slowly go down?
 

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You could try weighing down the pedal overnight. Sometimes even though you dont see any air bubbles during bleeding, there may be one trapped in the system. Having the pedal held down overnight will help let any trapped air migrate to master cylinder...
 

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Rear Reservoir (under right side engine cover)
1: Front Left Caliper Upper Bleeder Valve 8mm
2: Front Right Caliper Lower Bleeder Valve 10mm
3: Rear Caliper Lower Bleeder Valve 8mm
4: Anti-Dive: bleeder valve on top 10mm
5: Rear Caliper Upper Bleeder Valve 10mm


Do these two additional Steps to be sure you have no residual air bubble in the
rear system!!


6: Pressure bleeding the lower Junction block. (Upper left side of frame sitting on
bike).Follow rubber hose from Secondary master cyl to the Junction block.
7) Pressure Bleed the top Banjo bolt on the Secondary Master Cyl. (To be sure
the air bubble is completely out)
 

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I also had a brake bleeding problem on my Africa Twin AS with abs brakes. Turned out that after bleeding the brakes multiple times and at least two gallons of brake fluid that I had a very slightly bent rotor. This occurred after a tire change.
 

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Try this. You will need an assistant.

Top off the rear master cylinder. Have assistant pump pedal several times and hold pedal down. Open the bleeder valve on the secondary master cylinder and place a hose and catch bottle on it. When the pedal drops have the assistant pump the pedal as rapidly and as vigorously as possible a good 5 times. After the 5th time, hold the pedal down and close the bleeder.

This will probably blow the air bubble out the bleeder.

Next time you change fluid wet flush. Don't empty the system unless it is contaminated.
 

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Discussion Starter #7
Brake bleed issue

A bit of clarification on the symptoms you're observing:

1.) Immediately after bleeding the brakes, as you have done; is the Brake Pedal firm when you first apply pressure with your foot/hand, just (as in immediately) after you have closed the Bleed Screw(s)?

2.) Without taking any pressure off the Brake Pedal as described in Step 1 above; if you continue to hold a fairly constant pressure on the Brake Pedal for one minute (or even a bit more); does the Brake Pedal hold position, or does it slowly go down?
To #1, no pedal is soft and requires pumping to firm up.
To#2, after pumping the pedal to firm up, the pedal will slowly sink under pressure.
 

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Actually sounds like a bad master cylinder assuming you do not have a leak.
 

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It could be air, but bleeding the brakes isn't rocket science and it shouldn't require any special procedures. I'd be suspect of the master cylinder. It sounds like brake fluid is leaking back into the master cylinder when the pedal is depressed.

chrisashbrook beat me to it ;)
 

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Has the brake recall been done? If not, it may be the ticket to a repair. If it has been done, call up the dealer that did the work and inform them of the failure. Because it sounds like a master cylinder not holding pressure during application.
 

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The brake pressure builds up while pumping the brake pedal but creeps down with gentle pressure on the pedal... this is a textbook symptom of a bad master cylinder. The brake fluid is bypassing the seals on pressure piston internally. There will be no leak found and no bubble or foaming of the fluid.
 

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To #1, no pedal is soft and requires pumping to firm up.
To#2, after pumping the pedal to firm up, the pedal will slowly sink under pressure.
The Primary Piston Cup Seal in your rear Master Cylinder is leaking. Normally: disassembly, a light hone, thorough cleaning and re-assembly with new seals will take care of things. However, the bore of the Master Cylinder; after performing a light hone and thorough cleaning, must be absolutely perfect. If the bore isn't perfect, a replacement Master Cylinder will be required.

If a rebuild is possible; Honda Part Number 43520-MCA-016 will have all of the parts and seals needed. Probably about $27-$28 On-Line. A complete new Master Cylinder will run about $135. Honda lists two different Part Numbers for your 2008. It is possible that either Part Number (as a complete replacement assembly) will be fine. However you'll probably want to take a look at your existing Master Cylinder's markings, in the unlikely possibility, that the Part Number is stamped or cast into it. Replace the one you have with the same part Number if you can. If there is a difference (which I kind of doubt) it is probably in the linkage and not anything internal to the Master Cylinder's Hydraulics. The two Part Numbers are: 43510-MCA-016 and/or 43510-MCA-305
 

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Discussion Starter #13
Master Cylinder rebuild

The Primary Piston Cup Seal in your rear Master Cylinder is leaking. Normally: disassembly, a light hone, thorough cleaning and re-assembly with new seals will take care of things. However, the bore of the Master Cylinder; after performing a light hone and thorough cleaning, must be absolutely perfect. If the bore isn't perfect, a replacement Master Cylinder will be required.

If a rebuild is possible; Honda Part Number 43520-MCA-016 will have all of the parts and seals needed. Probably about $27-$28 On-Line. A complete new Master Cylinder will run about $135. Honda lists two different Part Numbers for your 2008. It is possible that either Part Number (as a complete replacement assembly) will be fine. However you'll probably want to take a look at your existing Master Cylinder's markings, in the unlikely possibility, that the Part Number is stamped or cast into it. Replace the one you have with the same part Number if you can. If there is a difference (which I kind of doubt) it is probably in the linkage and not anything internal to the Master Cylinder's Hydraulics. The two Part Numbers are: 43510-MCA-016 and/or 43510-MCA-305
Thanks so much for the diagnosis and part number information. I will be ordering a rebuild kit through Partszilla as soon as I check the markings on the cylinder. The repair looks pretty straight forward. This master cylinder replaced the original when I had the recall done about 2 years ago. Thanks to everyone that offered their advise and comments. I'll update when the repair is complete.
 
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