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I heard somewhere that bikes with hydraulic brakes should not be put upside down. If they are, 15 minutes should be allowed, before the bike is used.

Is there any truth in this? What process could be happening.

I am asking this, because I sometimes leave my bike upside-down overnight, to remove any water from inside the frame. Today I noticed some oil on the inside of the lever of my BR-M445L.

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If you find that you do this often then you should consider making your frame waterproof (i.e sealing any possible water bottle cage holes with silicone or something) – cherouvim Dec 17 '13 at 9:54
Or drill a small hole in you BB shell. That's absolutely no problem on an aluminum frame. – arne Dec 17 '13 at 10:01
Another reason to leave the bike upside down is to help relubricate the front fork seals. – James Morris Dec 19 '13 at 11:50
I found my problem. The reservoir cap on the lever leaks oil, when the bike has been upside down for a minute or more. Good news is, I bought two sets of dirt-cheep brakes and the other model is not exhibiting this behavior. Bad guy: BL-M505L, Good guy: BL-M486. – Vorac Apr 17 '14 at 7:24
Note that the leaking lever has another disadvantage: it is unwieldy for me. – Vorac Apr 17 '14 at 7:25
up vote 10 down vote accepted

There are two risks to turning your hydraulic brakes upside-down.

The brake system isn't filled to overflowing with hydraulic fluid: there's likely to be a small air bubble. Normally, this sits at the highest point of the system: the oil reservoir at the brake lever. There, it's not compressed by the piston when you brake, so it can't interfere with braking. If you up-end your bike, air bubbles might travel through the system to the brake pads. If they don't manage to make it back up to the reservoir before you want to brake, you might find your brakes are squishier than usual. 15 minutes is perhaps a pessimistic estimate, and it's not that much of a risk unless you get on your bike and right away need to brake very hard.

The other potential problem is if the seals in the oil reservoir aren't quite tight. You might not notice in normal use, because they're at the top, but when they're at the bottom of the system, the pressure might let some brake fluid seep out. If you notice this happening, stop turning your bike upside-down, and make sure to check that you still have enough fluid to brake properly. If it keeps happening after you've re-tightened the seal, you may need to change the rubber grommet(s). Often rubber seals are dissolved over time by the oil, and stop doing their job properly.

As others have said in the comments, you'd be better off avoiding the need to turn your bike upside-down. Find where the water is getting into the frame and seal any gaps. Some bikes already come with holes in the bottom in case they need to drain, and you might consider adding your own.

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Only if the brakes have been poorly bled or maintained, in which case you can run into the problems that Dan pointed out. If everything is as it should be, it should not cause any problems. If your brakes become squishy then they need to be bled. Air in the system is never a good thing.

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Down voted why? – joelmdev Apr 17 '14 at 14:23

In Dorenaz (CH), to go to the top of the mountain you must take a cable car for 5 min and the bike is upside down ... and then you ride a downhill trail where you need to brake really hard sometimes ... but I never have had a problem.

Why? Because if you have bled your brakes well when you service them there should be no air in the system :)

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