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Fluid Level In GRX / Ultegra Levers?

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Fluid Level In GRX / Ultegra Levers?

Old 12-05-22, 06:57 PM
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Steve_sr
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Fluid Level In GRX / Ultegra Levers?

I just finished installing a GRX group set and bled t he brakes according to Shimano's instructions including the angled lever burping. When I removed the funnel from the levers I was surprised to discover that there was still a significant amount of air in the top of the reservoirs. Is this as Shimano intends?

I don't really see any issues as long as the bike stays upright but what happens if the bike is laid down to transport in the back of a vehicle or if the bike is stored upside down hung by its wheels? Under these circumstances is air likely to enter the system?

Last edited by Steve_sr; 12-05-22 at 07:04 PM.
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Old 12-05-22, 07:15 PM
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If I remember my hydraulics theory correctly, the reservoir is semi- isolated from the service line, so air wouldn't migrate as you describe. Also the system lines are narrow and once full there's no room for air to enter unless some oil weeps out first. It's sort of like the grade school experiment where a narrow necked bottle is filled with water, then covered with a sheet of cardboard and inverted. If the cardboard is carefully slid out the water stays trapped in the bottle until/unless the bottle is shaken. Viscosity and serface tension combined with atmospheric pressure keep the water in and air out.

If air were able to flow freely between the service line and reservoir, you wouldn't need to bleed the line, rather than waiting for air migrate to the top of the reservoir by gravity alone. Other evidence of that is that the lever doesn't immediately drain if you disconnect the line. OTOH - it's possible that if you leave an open hydraulic system upside down long enough, oil might eventually weep out of the service line allowing air to enter.



So, tentatively (and only in theory) you're OK not to worry about that, (at least until the bike proves otherwise)
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Last edited by FBinNY; 12-05-22 at 07:32 PM.
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