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Mountain Gear

Technical Q&A: A Need to Bleed?

Master mechanic Lennard Zinn follows up on questions regarding hanging bike storage and what that means for hydraulic brakes.

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Dear Lennard,
In your answer about hanging mountain bikes, what about hydraulic brakes?  I heard not to hang your bike if you have them because the air would go to the caliper instead of the master cylinder.


Dear Roger,
I can’t believe I overlooked that; thanks for catching it. Obviously, hanging it right-side up by the frame creates no braking problems.

However, hanging it upside down is a poor idea because air can gravitate to the calipers.

If the bike is hanging vertically with the front wheel up, there is often no problem because any air bubbles would gravitate toward the lever, which would be where you’d want them. But on some systems, hanging by the front wheel can place the reservoir below the master cylinder and may introduce air into it.

And how about with the rear wheel up? Some systems are designed to have all air evacuated completely from the entire closed system when bled properly, and with such a system, it would be irrelevant if it’s hanging upside down.

More commonly, though, many modern systems do have air above the fluid in the lever’s reservoir, and as long as there is sufficient fluid in the system there is no problem. If air gets in the system while in use, it rises up to the lever, out through the metering hole in the master cylinder and up to the reservoir where it sits above the fluid and causes no problem. With such a system, if you flip over in a crash, you can find that you have no braking for a while once you get going again because air from above the fluid in the reservoir passed through the metering hole ahead of the piston and into the brake lines.

The metering hole is ahead of the piston so that fluid can flow down from the reservoir into the master cylinder until the piston is pushed in far enough that it closes off the hole, at which point it can now build pressure through the system and push the pistons in the slave cylinders to force the pads against the rotor.

It would seem that hanging the bike upside down (rear wheel up) could conceivably create the geometry such that the air bubble in the reservoir is lined up with the metering hole, in which case air could bubble up into the brake lines.

(For those to whom this is new, air in hydraulic brake lines, whether on your bike or in your car, is a bad thing because air, unlike liquid, is compressible. So when you pull your brake lever or stomp down on your brake pedal, if there is air in the brake lines the pressure can simply compress the air rather than pushing the brake pads and you won’t be able to stop.)

In practice, I’ve hung a lot of bikes with Shimano and SRAM hydraulic brakes (both of which have reservoirs in the lever that can have air above the liquid) upside down without noticing a loss in braking performance afterward. I would have to guess that normally the angle of the reservoir when upside down is such that the air bubble does not come in contact with the tiny metering hole because of the angle of the lever on the handlebar in three dimensions relative to horizontal. But if air were indeed to travel into the cylinder and into the brake lines, pumping the lever a few times once the bike was again upright should squeeze those bubbles, and, with the help of gravity, move them rapidly up to the master cylinder and up through the metering hole and out into the reservoir. At that point the brake would perform correctly again.

I doubt that in a static situation like hanging a bike, a bubble could make its way all of the way into one of the wheel (slave) cylinders, due to the various bends in the hydraulic tubes. Air bubbles generally are unimpeded coming back up to the master cylinder under repeated braking unless they get stuck in corners of a slave cylinder.

So, in answer to your question, hanging the bike by the front wheel will most certainly not compromise your hydraulic braking, but hanging it by the rear wheel might. And in most cases, the problem should clear itself up again soon with some vigorous pumping of the levers.


Dear Lennard,
I have a customer with Avid brakes who stores his bike hanging from the front wheel. And guess what? He often has spongy brakes! I cure it by removing the lever from the bar, holding so that the port is uppermost and pump the lever. But in regular use the “metering” hole, in the piston’s chamber, is never the highest point and air is trapped there. The solution is to bleed the system so that there is no air in the reservoir either, but I have difficulty doing this with the Avid. For fun, look at an old Hayes handpiece, they run “upside down” and will be “safer” stored hanging than any other position!

New Zealand (Down Under… how does that affect the situation?)

Dear Andrew,
Down Under you’ll need to do everything upside down from us up here.


Technical writer Lennard Zinn is a frame builder (, a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Follow Lennard on Twitter.