Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: AMuzi <am@yellowjersey.org>
Newsgroups: rec.bicycles.tech
Subject: Re: Bleeding Disc's
Date: Fri, 21 Mar 2025 10:30:30 -0500
Organization: Yellow Jersey, Ltd.
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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.
>>>>>
>>>>> 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.
>>>>>
>>>>>
>>>>> 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.
>>>>>
>>>>> 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.
>>>>
>>>> Right you can't purge the air unless fluid flow is 
>>>> bottom to top. Exactly the opposite of autos.
>>>>
>>>> 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.
>>>>
>>>
>>> We called that "physics" where I come from.
>>
>> Well, yes.
>>
>> 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).
>>
>> 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.
>>
> 
> 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