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Hydraulic disc brakes - loss of line pressure after changing tire?

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Hydraulic disc brakes - loss of line pressure after changing tire?

Old 10-10-16, 09:40 AM
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Hydraulic disc brakes - loss of line pressure after changing tire?

I just changed my front tire and tube last night. Went to ride it this morning and have zero front braking power. The same thing happened when I changed the pads. I had to send it to the LBS.

What gives? How do I fix this?

Only thing I can think of is that I don't have a repair stand, so I flip it upside-down to do the maintenance.
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Old 10-10-16, 09:52 AM
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consult the LBS.. where you bought the bike, may need bleeding or replacing, entirely..
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Old 10-10-16, 10:21 AM
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One of two causes.

1) Air bubbles in the lines/caliper. Most bike have a fluid reservoir at the lever where air bubbles tend to migrate, while in the reservoir they are harmless. Flipping the bike upside down and pulling the lever (even a tiny amount), can let those move into the lines/caliper.
A properly bleed brake system can be flipped upside down, but if it's not perfect you get issues.

2) Pulling the lever without a rotor or spacer in the caliper. This causes the pistons to over-extend, and not be centered properly. If the pistons extend too far they can actually leak fluid, leading to major braking issues. If there's fluid on caliper, its probably this issue.

The fix: Bleed the brakes.
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Old 10-10-16, 10:49 AM
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GSA hit on it. Did you accidentally pull either of the levers while the wheel was off the bike? If no, did anyone else? i.e. children, etc? If no, then there is probably air in the line. When you flipped the bike upside down air bubbles may have traveled up the line into the master cylinder by the lever. That will kill your brakes as the lever is compressing air and not pushing fluid to actuate the brake calipers. How the air got into the system is anyone's guess if the levers weren't pulled.

If you know how to bleed the brakes try a top and bottom bleed after you check the fittings then flip the bike upside down again and see if the problem returns. If it does, then you might have a compromised system. Check all of the fittings again and tighten them slightly. Re-bleed. Try it again. If still not working, yes, best to take it to a shop and explain the situation to them.

Last edited by drlogik; 10-10-16 at 10:53 AM.
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Old 10-10-16, 11:49 AM
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A bit of info, like what brakes you have would be useful, How are you changing the pad/wheel, if you are turning the bike over, so that it sits on it's handlebars/saddle, this could cause the air which will be in the reservoir to migrate to the hose/areas where it shouldn't be, which may be causing the issues you are getting?
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Old 10-10-16, 12:10 PM
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I don't have hyd brakes on any of my bicycles, but when you remove/reinstall a wheel on a motorcycle, you typically push the brake pistons slightly into the caliper and after reinstalling the wheel it takes a few pumps of the brakes to return the pistons to their former position and restore normal function. I have no idea if the same happens on bicycle, but I thought I'd throw this out there.

On any hydraulic braking system a big no-no is pressing the brake lever/pedal/whatever while the disc is removed. On a bicycle, I'd think this would be easy to do. A slight mistake usually just requires pumping the brakes to restore pressure, but if the piston was pushed far enough out to allow any air in, you need to bleed the brakes.

- Mark

Last edited by markjenn; 10-10-16 at 12:16 PM.
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