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Thread: Brake bleeding

  1. #11
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    Default Re: Brake bleeding

    I agree to most of your post but not to the part that says the air bubble can block the flow.
    Sorry, but you are wrong. If it couldn't block flow - he would have been able to push it out of the system in any orientation.

    Greg

  2. #12

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    Default Re: Brake bleeding

    I disagree.
    The fluid flow goes slowly around or under the bubble. If the bubble was blocking the flow, it would have moved when he was pushing the fluid up with the oil can or syringe.

    Ofer Gd

  3. #13
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    Default Re: Brake bleeding

    It does not - all it does is either move the air bubble and/or compress the air.

    Try not bleeding the air out of the high point of your radiator cooling system. The air block (called air lock) prevents the flow of fluid. See how long it takes for your engine to overheat! Please do it in your car though so your on the ground!

    Greg

  4. #14
    Senior Member jiott's Avatar
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    Default Re: Brake bleeding

    The kind of air lock you are talking about is due to the air pocket being pushed thru the system to the pump. At the pump you have trouble because the pump vanes/pistons/rotors/whatever depends on the fluid to seal up the clearances and gaps in order to create the pumping action. With an air pocket in the pump, it has trouble priming and will just spin without moving fluid. This brake system we are talking about has nothing to do with pumps since the bubble is a long way from the master cylinder (pump).
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  5. #15
    Senior Member Esser's Avatar
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    Default Re: Brake bleeding

    Greg is correct. Airlock in a high point of a pipe can block the flow. If you have enough pressure or enough flow it can move the airlock.

    Source: many years as a pipe fitter
    ------------------
    Josh Esser
    Flying SS7
    Rotax 914iS
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    Edmonton, AB, CWL3

  6. #16
    Senior Member jiott's Avatar
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    Default Re: Brake bleeding

    The kind of air lock you are referring to Esser is explained in this link: https://en.wikipedia.org/wiki/Air_lock. I don't believe this applies to a small bubble in the brake system we are talking about. Also heating of the bubble can cause localized expansion and hi pressure and block flow, but again I don't believe this applies to the brake system. Like I said before, I have had a small 3/4" bubble in my brake system near the reservoir for 3 years and it in no way affects the operation of my brakes. If it bothers anyone, get rid of it, but it is certainly nothing to worry folks about.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  7. #17
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    Default Re: Brake bleeding

    Wrong on all counts Jim. And for the record - I did not say it would affect your brakes. I was referring to Ken's much larger airlock (it's his thread) and replying to ofergd's question.

    I said it would move and/or compress the bubble - fluid will not pass by it. I at no point was referring to your situation. I agree that in it's location it will LIKELY not affect the braking as the bubble is not between the master cylinder and the piston.

    Further, with hard and/or continuous brake usage the brake fluid gets hot and expands which is most of the reason for a reservoir in the 1st place. If it can't get past the bubble (blockage) you could then experience issues.

    The kind of air lock you are talking about is due to the air pocket being pushed thru the system to the pump.
    Also not the case, yes a radiator/cooling system has a pump but it is low pressure maybe 14 to 15 psi - you are referring to pump cavitation - different issue entirely.

    Any high point in a fluid system with air in it can and will cause airlock. Many cars have high points above the pump and many feet away from it and they have bleeders there to get rid of the air. If not bled, the air blockage will prevent flow and it will overheat. Likewise the return spring on a hydraulic brake system creates pressure (like a coolant pump and likely at a similar pressure or higher) to release the brake pad and return the fluid to the reservoir.

    Source: 20 year plus retired fluid engineer/company owner.

    Cheers, Greg

  8. #18
    Senior Member jiott's Avatar
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    Default Re: Brake bleeding

    Greg you did say the bubble we are talking about could block the flow of fluid. In this case it clearly does not because we observe the bubble moving back and forth with brake application. Your statement "any hi point in a fluid system with air in it can and will cause airlock" is totally untrue. That link I referred to explains that it only occurs under certain conditions. Anyway this discussion is turning into something that does none of us any good, so I will respectfully agree to disagree and end it for me at least. The ONLY way that bubble we have been discussing could cause any trouble is if your brakes leak and you continually push fluid out onto the ground, then it might be gradually drawn down into the master cylinder and cause sponginess.

    Source: 40 years retired hydraulics engineer/not company owner.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  9. #19
    Senior Member jrevens's Avatar
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    Default Re: Brake bleeding

    I agree with Jim. In this instance there will be no "blockage" from this bubble. The air is more compressible than brake fluid, so it might have to compress a tiny little bit before the fluid between it and the reservoir moves, depending on how much head pressure there is in that part of the system, but that is probably unlikely. The bubble will just move back & forth with the fluid, and maybe a little fluid will bypass the bubble along the wall of the tubing, as ofergd suggested. I did study fluid mechanics in-route to getting my engineering degree, but I claim no "expert" knowledge - just experience with similar aircraft brake systems, and sometimes a little common sense.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  10. #20
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    Default Re: Brake bleeding

    For some reason, I'm thinking about air in a fuel filter. Doesn't stop fuel from flowing...... I almost typed this at least three different times but didn't. Jim Chuk

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