Re: Bleeding Disc's

On 3/21/2025 9:54 AM, Zen Cycle wrote:

On 3/21/2025 9:00 AM, AMuzi wrote:

On 3/21/2025 4:08 AM, zen cycle wrote:

On 3/20/2025 4:53 PM, AMuzi wrote:

On 3/20/2025 2:46 PM, cyclintom wrote:

I can't say what's going on. The fact that you had to bleed the rear brakes backwards but I cured the front brakes which you would think wouldn't be a problem since it is all vertical hoses was amazing. Bleeding that brake from top to bottom releases a lot of air bubbles after which the disk levers were hard and with about a half way pull to lock.
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The levers with the bleeding cap off appeared top be full of liquid so the levers were full and it couldn't be air trapped somewhere in the levers.
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but installing a funnel and filling it half way and using the syring on the bottom pulled a great deal of air out of the system.
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So while the Shimano instructions and most of the videos showed bottom to top, bleeding trom top to bottom as one video, made by a woman mechanic seems to be the only way to get the brake properly bled.

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Right you can't purge the air unless fluid flow is bottom to top. Exactly the opposite of autos.
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Air collects in the various caliper voids and doesn't readily reach the line when you are pushing fluid. Remove the caliper from its mount and tap it with something while you flush the line.
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We called that "physics" where I come from.

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Well, yes.
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In autos, the system is designed to purge with downward flow. Bicycle systems have voids which are prone to air pockets in inconvenient position and orientation. Also, the line diameter in relation to fluid viscosity is unfavorable (air bubbles tend 'stick' in place, hence 'tapping' to free them).
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Anyone experienced in auto or motorcycle clutch and braking hydraulic systems will have to learn bicycle systems from scratch; they are very different in application despite the same physics in both.
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 ISTM a major factor in designing automobile hydraulics to facilitate top-to-bottom purging is that it isn't practical to flip the car to different orientations.
 

+1
I didn't take notice for many years, but I now find that upside down contortions, while juggling my glasses and a light, have made auto maintenance extremely uncomfortable.
--
Andrew Muzi
am@yellowjersey.org
Open every day since 1 April, 1971