Re: RE: RE: Re: GRX Bleeding.

On 3/3/2025 11:10 AM, cyclintom wrote:

On Mon Mar 3 16:57:58 2025 cyclintom  wrote:

On Sun Mar 2 11:20:56 2025 AMuzi  wrote:

On 3/2/2025 11:16 AM, cyclintom wrote:

I have no leaks anywhere but the rear GRX in particualr will not bleed correctly. I will try tilting the bike in the stand so that any air in the system will find its way to the lever. The Tektro set of brakes I had had this problem but leaving it lay overnight allowed air bubbles to work their way out of it and it worked fine after that.

>
Remove the rear caliper so it hangs well below the rest of
the system. Rap the caliper lightly while you're pushing
fluid through, up to the lever.  The way the rear caliper is
mounted, the line dips down from the caliper before rising
up to the lever. Air in the caliper won't move without some
encouragement

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After thinking about it I realized that on all of my previous disc bikes that the line ran up the \seat stay and down the top tube to the nevers and the actuators were mounted on the back of the seat stay. This gave pretty much an alway "up" route for the brake fluid making it easy to flush air out of the system. The Idol has internal brake hoses making a lot of ups and down and making it difficult to flush air out of the system. I did try using the rear actuator port for pumping air out of the system and that pulled out a lot of air. But the brake still would not pump the brake pads in very colose to the discs and it took full pull to get brakes though they were pretty strong.. Now I have to figure out how to eliminate all of the lever travel.

    And considering the Tekitro system - it uses regular brake fluid which is a LOT less viscus so that would make air bubbloes move more freely upwards which is why leaving it overnight allowed the brakes to work fine the next day. Shimano's mineral oil has a high viscosity in comparison which is prolematic.

Well, yes.
More significantly, in all our bicycle hydraulic systems the internal area of the lines is smallish relative to the fluid viscosity and so air bubbles will not migrate up without assistance in most cases. In practice, that's very different from automotive brake lines.
--
Andrew Muzi
am@yellowjersey.org
Open every day since 1 April, 1971