No rear brake on my F4 - MVAgusta.net
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Old 09-18-2017, 06:35 PM Thread Starter
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No rear brake on my F4

I recently changed my rear brake to an aftermarket replacement and it seemed to be working fine. Then one day there is zero brake function. I bled the brake and managed to get good pressure, rode the bike and it was ok. Came back a few days later and again no brake. I mean zero pressure. I looked for signs of fluid but nothing so before I bleed it again anyone have any ideas.
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Old 09-18-2017, 06:43 PM
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Hmmm...Sealed system. No leaks. Where does the reservoir port into the master cylinder? Vertical or to one side? Bleed again and pay attention to volume of bubbles. A lot or just a tiny amount?

You didn't use a pneumatic bleeder did you? High line pressures will generate microscopic bubbles in the fluid which will join together after a short time to create mushy brakes. Most pneumatic bleeders call for less than 90 psi.

I used to be fast....now I just dream about it.
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Old 09-18-2017, 09:17 PM Thread Starter
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I removed the caliper and held it higher than the master cylinder, pumped the pedal and released the pressure, then repeated until there was pressure. Will do it again
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Old 09-19-2017, 12:12 AM
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George;
Remove the caliper, hold it low with the line fitting at the high point, push the pistons all the way back in
Remount caliper

Now all the air is in the MC, now loosen or remove a bolt on the MC and rotate it so the pushrod end is highest
Push the lever repeatedly, slowly, the piston will expel all the air through the return port

Been doing it like this since I was a kid


Think like a bubble

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Old 09-20-2017, 03:11 PM Thread Starter
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I understand the bubble reference which is what I was trying to do. If there was air in the line it should have been removed. Still cant figure out why all the pressure went away
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Old 09-20-2017, 03:46 PM
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If the piston seal is leaking by from the pressurized to the unpressurised side it will feel like no pressure.
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Old 09-21-2017, 07:16 AM
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Quote:
Originally Posted by silentservice703 View Post
If the piston seal is leaking by from the pressurized to the unpressurised side it will feel like no pressure.
Yep....if no fluid leak and properly bled then the master cylinder cups are not sealing.

I used to be fast....now I just dream about it.
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Old 09-21-2017, 08:17 AM
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Have you removed the dust boot? If the piston seal is leaking you'll find fluid behind the boot.

I've got a big bag of copper washers. Let me know if you need some George

.

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Old 09-21-2017, 08:26 AM Thread Starter
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I took the master cylinder apart last night and am having trouble figuring out how air is removed from the piston cavities. Fluid goes in one side and is supposed to flow to the other cavity but there is no place for the air to escape or be bled off other than where it enters. I don't know, maybe I am missing something since I am not an engineer.
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Old 09-21-2017, 01:48 PM
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George...this explains everything, even if it is a car drum system described, the master cylinder piston works the same way.


I used to be fast....now I just dream about it.
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Last edited by esq'z me; 09-21-2017 at 01:53 PM.
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