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  #1  
Old 01-02-2019, 09:38 PM
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Eliminating Trapped Air

I've had good luck removing air trapped in the front brake system of some bikes by turning the handlebar and tieing off the lever. But I'll admit that I'm not sure why this works. Does the extra pressure in the system squeeze the air and force it out of where its stuck, and then it can rise to the highest point (i.e., the master cylinder)? Just curious.
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Old 01-03-2019, 12:17 AM
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Re: Eliminating Trapped Air

As I understand it, tying off the brake lever keeps the master cylinder "open" and any air in the line naturally rises and is able to release into the master cylinder - something I've always done when I've bled brakes on any bike.
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Old 01-03-2019, 09:07 AM
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Re: Eliminating Trapped Air

I think it also compresses the air bubbles in hopes of dislodging them from whatever surface they might be clinging to.

One thing to look for is whether after zip tying the lever overnight is whether the lever remains at full travel or if it can be pulled back even more. If it still is not at full travel, there maybe a leak past the internal seals.
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Old 01-03-2019, 11:20 AM
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Re: Eliminating Trapped Air

Good way to see if there are any leaks.Should be SOP after any brake work that involves cracking open banjos.....line replacements,iAbs removal on old BMWs etc....!

If air is trapped in the MC only there is a rather quick way to bleed it out via the pressure release hole at the bottom of the MC.
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Old 01-03-2019, 01:21 PM
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Re: Eliminating Trapped Air

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Originally Posted by pbegin@burton
If air is trapped in the MC only there is a rather quick way to bleed it out via the pressure release hole at the bottom of the MC.
Where is that pressure release hole exactly? And whats the procedure? Does brake fluid leak out when it's opened?
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Old 01-03-2019, 02:05 PM
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Re: Eliminating Trapped Air

There was a change in model year 2013 where BMW eliminated the bleed screw at the MC. I was able to get the air out using the old-fashion method of turning the handlebar to the left and holding the brake leaver back overnight. I think I now have all the air out.
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Old 01-03-2019, 02:37 PM
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Re: Eliminating Trapped Air

Quote:
Originally Posted by bobby
Where is that pressure release hole exactly? And whats the procedure? Does brake fluid leak out when it's opened?

The relief hole is semi hidden...laying on your left side, with your legs in a stair climbing position, generally lets out excess air. Periodically, some brake fluids will seep out as the air releases.
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Old 01-03-2019, 03:23 PM
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Re: Eliminating Trapped Air

Quote:
Originally Posted by bobby
Where is that pressure release hole exactly? And whats the procedure? Does brake fluid leak out when it's opened?

Most if not all MCs have two holes at the bottom of the reservoir.Bigger one sends fluid into the piston/lines etc.Tiny hole on the lever side is pressure relief.

Apply brake but just until you feel some resistance.1" or 2" of lever travel? Very slowly release the lever and watch the stream of tiny bubbles come out of the pressure relief hole.Wait a few seconds, 20? And repeat....!
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Old 01-03-2019, 04:12 PM
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Re: Eliminating Trapped Air

Quote:
Originally Posted by pbegin@burton
Apply brake but just until you feel some resistance.1" or 2" of lever travel? Very slowly release the lever and watch the stream of tiny bubbles come out of the pressure relief hole.Wait a few seconds, 20? And repeat....!
Worked perfectly. Fluid change complete! Thanks all.
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Past BMWs
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  #10  
Old 01-03-2019, 07:16 PM
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Re: Eliminating Trapped Air

Quote:
Originally Posted by bobby
I was able to get the air out using the old-fashion method of turning the handlebar to the left and holding the brake leaver back overnight. I think I now have all the air out.




Good to hear
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Old 01-03-2019, 09:06 PM
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Re: Eliminating Trapped Air

Quote:
Originally Posted by bobby
Worked perfectly. Fluid change complete! Thanks all.


Any bubbles out of the tiny port? I pulled a couple out of mine today.May have been more as I didn't remove the cover to look at first.

Expected that as I flipped the whole MC over a few weeks ago for general housekeeping.Could be a little more air in there,bike isn't passing the GS911 bleed pressure test unless I pump the brakes first.

Mind you the not passing the test may have something to do with the pads not fully seated as I cleaned the pistons recently.Happened before that sort of failed test so for now lever is tied to fully seat the pads and let air float up if any.
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