Mystifying Brake Bleed issue on an F800GT after installing brake line extension

[Claudio]

I have a lot of experience bleeding brakes on a variety of bikes including the infamous servo-based R1150RT. I've also replaced lines on the oilhead and bled the brakes no problem.  I am using the Haynes manual to guide the process on the F800.

 

Mystifying Behaviour on the F800GT.

Well, I am mystified by some stubborn air that got into my wife's new 2019 F800GT (ABS) after installing bar risers and a brake line extender from MV Motorad.

 

The Bike

The bike is stock except for two newly installed speed bleeders on the front brakes. 

It has ABS and I have a GS 911 to invoke the ABS pump (part of the bleeding process).

 

What I did:

1. Drained the master cylinder (front) and installed the extension with new brass washers at each coupling/banjo bolt.

2. My wife accidentally applied the front brake while I was adjusting the bar position ("no worries I thought" I have to do a full bleed anyway, maybe this introduced air that is difficult to evacuate?).

3. Added fluid to the reservoir and tried.a multi-vac to extract most of the air (pulled through about a 250 ml of fluid each side).

4. Started bleeding the left calliper first, then the right.

5. Actuated the ABS pump using the GS-911 and repeated steps 3-5 twice.

6. In all, put through about 500-750ml of fluid through the system.

 

 

Symptoms/Issue

1. I can't get any pressure at all built up on the front brake indicating there is a mass of air somewhere that hasn't moved. Fluid does move through the system but no significant observed at the bleeder valves.

2. No leaks/seepage detected anywhere. 

2. When bleeding using speed bleeders I get a tiny bit of fluid at the very end of the lever compression. Closing them and trying to build up pressure before releasing does nothing.

3. I tried the "clamp the lever overnight" to see if some air would settle out...no effect.

 

Suspicions

1. Do I have air right at the master cylinder that I can't seem to evacuate? There is no bleeder valve directly on the master but is there a technique I am missing?

2. Should I be removing the speed bleeders for the initial bleed.

 

Starting Over

1. Would the best approach be to vacuum pump out the entire system and start anew at this point?

 

Other advice?

[dirtrider]

15 hours ago, Claudio said:

I have a lot of experience bleeding brakes on a variety of bikes including the infamous servo-based R1150RT. I've also replaced lines on the oilhead and bled the brakes no problem.  I am using the Haynes manual to guide the process on the F800.

 

Mystifying Behaviour on the F800GT.

Well, I am mystified by some stubborn air that got into my wife's new 2019 F800GT (ABS) after installing bar risers and a brake line extender from MV Motorad.

 

The Bike

The bike is stock except for two newly installed speed bleeders on the front brakes. 

It has ABS and I have a GS 911 to invoke the ABS pump (part of the bleeding process).

 

What I did:

1. Drained the master cylinder (front) and installed the extension with new brass washers at each coupling/banjo bolt.

2. My wife accidentally applied the front brake while I was adjusting the bar position ("no worries I thought" I have to do a full bleed anyway, maybe this introduced air that is difficult to evacuate?).

3. Added fluid to the reservoir and tried.a multi-vac to extract most of the air (pulled through about a 250 ml of fluid each side).

4. Started bleeding the left calliper first, then the right.

5. Actuated the ABS pump using the GS-911 and repeated steps 3-5 twice.

6. In all, put through about 500-750ml of fluid through the system.

 

 

Symptoms/Issue

1. I can't get any pressure at all built up on the front brake indicating there is a mass of air somewhere that hasn't moved. Fluid does move through the system but no significant observed at the bleeder valves.

2. No leaks/seepage detected anywhere. 

2. When bleeding using speed bleeders I get a tiny bit of fluid at the very end of the lever compression. Closing them and trying to build up pressure before releasing does nothing.

3. I tried the "clamp the lever overnight" to see if some air would settle out...no effect.

 

Suspicions

1. Do I have air right at the master cylinder that I can't seem to evacuate? There is no bleeder valve directly on the master but is there a technique I am missing?

2. Should I be removing the speed bleeders for the initial bleed.

 

Starting Over

1. Would the best approach be to vacuum pump out the entire system and start anew at this point?

 

Other advice?

Morning  Claudio

 

It sounds like the master cylinder is air bound so won't initially make enough pressure to push the air through the system  (especially with those speed bleeders in place)

 

Personally, I would start by bench bleeding the master cylinder (but, just do that with the master cylinder still on the motorcycle).

 

Fill the master cylinder reservoir, then move the hand lever very slightly (just a few millimeters) keep doing this until you get most of the air in the master cylinder bore to come back UP & out of the small take-up port in the top of the master cylinder.  

 

Once you get enough fluid into the master cylinder bore that you are not pumping/compressing ONLY air then you can probably get it to bleed. 

 

You might also try removing those darn speed bleeders, then filling the master cylinder reservoir, then try using your mouth to blow pressure into the reservoir to force the initial slug of  fluid into the master cylinder bore & partially through the system.  (this requires the speed bleeders to be removed)

 

Once you get the master cylinder mostly air free then the system will probably bleed OK using the hand lever. 

 

If it STILL won't bleed for you then you might have to force fluid back up through the system from the bottom (from calipers)  to push fluid through UP through from the bottom as that will push most of the air up & out on the fluid column. (again speed bleeders removed)

 

 

[greatpav]

Thank you for the knowledge you have shared. Looking forward for more contents similar to this.

[Michaelr11]

I like using speed bleeders when the system is filled and performing a typical maintenance fluid bleed.  They won't work properly when the system is air bound.  You can't back fill the lines with speed bleeders in place.  Put the original bleeders back in place until you have the system filled and you get pressure back at the lever.

[BrianM]

I agree with DR. Don’t need speed bleeders.

 

Attach tight fitting hose to standard bleeder. Loop hose up a few inches and down into container. Put hand on lever or pedal. Crack open bleeder just enough to get fluid to flow. Pump lever/pedal until reservoir nearly empty, leave bleeder open this whole time. When reservoir near empty, as lever/pedal near end of stroke, close bleeder. I like to close bleeder while fluid is still moving.

 

I do the same on cars and trucks. Just need a second person.

[Claudio]

Thanks for the advice everyone.  I think the likelihood that the master cylinder is retaining that air is the cause.  I'll try the bench bleed technique.

 

 

[Claudio]

Just a follow-up and thanks. 

 

The "bench bleed" feathering of the master cylinder did the trick.  Lots of air bubbles were released and I was able to easily beed the system easily without any further mystery.

 

Thanks again.

[Tmuile602]

Great information here.  I spent a few days messing around with a F800GT, trying to get some feel in the front brake lever.

Feathering / flicking the front brake lever let the air bubbles come up through the master cylinder.  I was able to get some feel in the lever with the bike on the center stand and the bars turned to the left.  I eventually put the bike on the side stand with the bars turned to the left and saw a bunch more air come out.  Now the lever feels normal.  I'll let it sit overnight and trying it again tomorrow to see if more air comes to the top.