Can't get air out of front brake - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 23 (permalink) Old 06-22-2015, 04:12 PM Thread Starter
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Can't get air out of front brake

I should have changed brake fluid earlier but one of the master cylinder screws was siezed...but it was becoming apparent that the front brake had become spongy and I needed to address the issue. I got an impact driver to get the screw loose without stripping it, and found the fluid was real muddy, with sediment in the reservoir. When draining the system I accidentally allowed the reservoir to empty so that the inlet port in the master cylinder was exposed to air. Figured no big deal, I'll just bleed the system.

I fist bled the master cylinder by cracking the nut at the discharge hose under pressure and squeezing the hand lever until no air bubbles came out, then retightening the nut under hand lever pressure. Then I started bleeding the wheel calipers.

My procedure: remove MC reservoir cap, top off reservoir, replace diaphragm as a splash shield (otherwise when the lever is pressed it gushes a little fountain of brake fluid onto the tank), place 10mm box end wrench over bleed screw, attach clear vinyl hose to bleed screw, other end of hose in small reservoir of used fluid, depress brake lever, open bleed screw, watch fluid in hose, close bleed screw, release lever. Got a few air bubbles out this way but it's real time consuming since the volume in that little reservoir is small so I keep having to stop and refill it.

Since then I have been bleeding the wretched thing for hours. I have run nearly a quart of DOT4 through with no improvement in the sponginess. Fluid is clear and colorless now but I keep seeing really small, fine bubbles in the clear hose, pinpoint size, just a few with every depression of the brake lever, but they don't quit. I have tried gravity-bleeding and also pumping the lever 12-14 times with the bleed screw open and then go back to the pressure-release method but this does not help. When bleeding under lever pressure I still see these small air bubbles. A dark background is necessary to see them but they're there.

I have checked all fittings for tightness and there are no leaks. I tried reverse-bleeding the system and fabricated a squeeze bottle for that purpose but found no way to attach a hose to the bleed screw without entrapping an air bubble which would just be forced back into the system. I can't see how buying a mityvac or its Harbor Freight equivalent will be helpful since I've already run so much fluid through the system.

I can see that if the fluid has a lot of very small bubbles it might be compressible enough to cause sponginess, but where the heck are they coming from? I wonder if the pinpoint bubbles are real or just something that happens at the bleed screw, and that the master cylinder is the cause of the sponginess since there was a fair amount of muck in the system. I could disassemble everything and rebuild but since the brake is operable, just spongy, that may be a waste of time.

Haven't tried the hold-lever-open idea yet but am open to suggestions.

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post #2 of 23 (permalink) Old 06-22-2015, 09:02 PM
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Well, haven't had the 'joy' of brake problems yet. Not sure I can be much help but I can give you a bump.

Also, if the fluid is thick it will hold bubbles more readily. Maybe try heating the fluid some before bleeding them again. I see you live in Arizona. Just leave a bottle in the sun for a while and the heat will thin it down and make it less viscous. Maybe then the air bubbles will not be as readily trapped or will escape the fluid easier. Just a thought.

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post #3 of 23 (permalink) Old 06-23-2015, 08:55 PM Thread Starter
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warm brake fluid

I kind of had the same thought. Except the last time I bled the brakes it was 100+. Plenty warm enough.

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post #4 of 23 (permalink) Old 06-23-2015, 10:43 PM
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Wow. Can't believe more people haven't chimed in with suggestions. Maybe, have you tried the Facebook page? People get notifications there when a post is made.

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post #5 of 23 (permalink) Old 06-24-2015, 06:37 AM
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Quote:
Originally Posted by sgsibob View Post
Haven't tried the hold-lever-open idea yet but am open to suggestions.
Pump the lever and tie it down. Leave it over night. That is the only way I was ever able to get it to firm up.

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Leave it to me to NOT be fond of something then end up banging it.
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post #6 of 23 (permalink) Old 06-24-2015, 10:07 AM
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Quote:
Originally Posted by Deviate View Post
Pump the lever and tie it down. Leave it over night. That is the only way I was ever able to get it to firm up.
X2. On that

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post #7 of 23 (permalink) Old 06-24-2015, 03:52 PM
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Quote:
Originally Posted by Deviate View Post
Pump the lever and tie it down. Leave it over night. That is the only way I was ever able to get it to firm up.
I haven't done anything with the brakes. Just curious - how does leaving the level "squeezed and tied" help firm it up? What does it do?



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post #8 of 23 (permalink) Old 06-24-2015, 05:03 PM
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The bubbles come to the top and disperse. I would leave the cap off too

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post #9 of 23 (permalink) Old 06-24-2015, 05:29 PM
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Yup. I didn't think it would be necessary either, but it really was the only thing that worked for me.

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post #10 of 23 (permalink) Old 06-24-2015, 09:28 PM
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Quote:
Originally Posted by Deviate View Post
Pump the lever and tie it down. Leave it over night. That is the only way I was ever able to get it to firm up.
Quote:
Originally Posted by OleDirtyDoc View Post
X2. On that
Quote:
Originally Posted by OleDirtyDoc View Post
The bubbles come to the top and disperse. I would leave the cap off too
Quote:
Originally Posted by Deviate View Post
Yup. I didn't think it would be necessary either, but it really was the only thing that worked for me.
Brilliant. And seems like it would work. Just takes for ever for the bubbles to float up. Why didn't I think of that??? Good job guys...........

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