Brake bleeding

[ken nougaret]

I pushed 5606 up both brakes 3 times each side alternating sides. I'm still left with this 2" section of air. I used the oil can and the syringe with the same results. Thoughts?

[HighWing]

Ken,
I'm not sure what I am looking at. My lines go up through the bottom fabric then across the floorboard near the cowl side then on the floorboard around the corner then up to the brake. In bleeding all is uphill from the caliper - especially with tailwheel. I don't understand the line to the top of the tube then back down.

[Dave S]

Ken,

Wondering if the orientation (line entering from the top of the fitting rather than the side or bottom) of the blue "T" could be the cause of the issue? If there is a large enough void inside the "T" could it possibly be holding a air bubble while allowing the more dense fluid to slip past into the reservoir? Just a thought.

Before making a lot of work of it, I'd go back to the brake wheel cylinder this line is connected to and give it another go. I found it useful to hook up a spillover line to the top of the reservoir and then not worry about excess fluid spillover. Possibly a little more forceful application of the oil can or syringe may blow the bubble into the reservoir out of the line? Another experiment may be to have two people applying a syringe to both sides at the same time.

[ken nougaret]

Lowell, your question prompted me back to the manual to take a picture for you. And Dave I suspected the same as you. After looking at the pic again, I see that I have the tee upside down. How did I do that? Thanks for the quick reply. I will flip the tee over and try again.

[ken nougaret]

Success! And I feel stupid. Thanks again, Ken

[Danzer1]

Ken,

I wouldn't feel stupid (but I know what you mean). You pointed out a few good things for everyone.

1. The value of asking questions as opposed to willy nilly going at it and getting frustrated with it and wasting time trying to figure it out or worse, leaving it and moving along.

2. The value of experience here and on a few other forums.

3. The value of at least one extra sets of eyes.

4. The value of "a picture is worth a thousand words".

Stupid would have been to not ask or doing it wrong twice!

Show me someone who hasn't made a mistake and I'll show you a liar!

Sounds like your getting closer, can't wait to see her done!

Greg

[ofergd]

What difference does air near the reservoir make? as long as it doesn't make it to the master cylinder.
The important part of bleeding the brakes is (if I understand correctly) that no air is in the lines between the wheel pistons and the master brake cylinders. This makes ''mushy'' brakes.

Ofer Gd

[Danzer1]

I'll take a stab at this:

Brake fluid is relatively incompressible, but it does expand as it gets hot. Also the fluid behind the master cylinder moves in both directions. Sucked towards the master cylinder when the brake is applied and back to the reservoir when the brake is released.

It is possible to suck the air pocket into the cylinder (not likely though).

The air pocket could also block the return flow of fluid to the reservoir, causing the brake to not fully release (also not likely). That is more likely though if the fluid heats up and expands and has no where to go.

Most likely though, the air pocket will just move back and forth in the line - I wouldn't count on it though! As Murphys Law says the time that doesn't happen is the one when you have a hot landing to far down the runway!

No air in the brake system - except in the space on top of the fluid in the reservoir where you fill and check level.

Greg

[ofergd]

I agree to most of your post but not to the part that says the air bubble can block the flow.
I definitely agree that these problem should be solved (and was) but if the line between this fitting to the cylinder is long enough and the fluid does not get to the cylinder when the brake is depressed, there is no real problem.

Ofer Gd

[jiott]

I also seem to have a small bubble of air in one of the lines right at the reservoir. It wasn't there to begin with (3 years ago) but seemed to slowly form over time. I suppose I should rebleed the brakes and get rid of it. When I first noticed it I applied both brakes by hand while watching the bubble up under the dash. It moved down some but nowhere near the master cylinders. Since the brakes have always felt fine, I quit worrying about it, taking the attitude of ofergd. However, I agree with Danzer, it is best to get rid of it and I will one of these days when I am working on that part of the aircraft.

[Danzer1]

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

[ofergd]

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

[Danzer1]

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

[jiott]

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).

[Esser]

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

[jiott]

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.

[Danzer1]

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

[jiott]

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.

[jrevens]

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. ;)

[avidflyer]

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

[Danzer1]

Jim, I agree we need to end this because obviously you do not comprehend what I am stating. I have stated more than once - the bubble will move - also that the bubble/airlock/blockage - call it what you want - will prevent the fluid from passing to the other side of it.

Don't continue further or I'll tear into your ill conceived notion of what constitutes "opinion" regarding your baseless hearsay claims against an engine manufacturer. I'm fed up.

Greg

& DF4 - I've been trying to be nice here, so no need to PM - I'm done.