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Air bubbles in brake line

647jc

647jc

Well Known Member
I have installed and bled the standard Van's dual brake system and it appears to be working ok however, when exercising the brakes I occasionally see an air bubble materialize in the semi transparent tubing between the fluid reservoir and the copilot brake cylinders. The standard brake installation has you use the nylon tee fitting at the reservoir rather than metal, I assume because it is low pressure at that point. I?m wondering if the vacuum or suction created in that line when pressing the brake pedals is sometimes strong enough to suck air through the nylon threaded fitting rather than fluid from the reservoir. I don?t have any fluid leaks but that doesn?t mean I couldn?t have a ?sucking air leak? ;). Has anyone else seen this phenomenon?
 
Brakes

Joe,
Keeping bleeding them from the bottom up. Mine took about 3 times to get all the air out. This is very common.
 
I really don?t think continued bleeding will help. The system has been bled a number of times and as I originally explained, the air bubble(s) only materialize in the tubing between the copilot cylinder and the reservoir. never between the copilot and pilot cylinders. This implies the only place the air could be hiding (if it were a bleeding problem) would be in the copilot cylinder. Typically when the bubble appears in the line it is within an inch or two of the nylon tee fitting on the reservoir thus I suspect I am sucking air in through the nylon threaded connectors. I haven?t really torqued the nylon nuts very much because I don?t want to strip them but I think I will try tightening them a bit more and see if that cures the problem. I was really wondering if anyone else had seen this exact same phenomenon.
 
Joe,
I have noticed bubbles also. My brakes have been bled twice already. I think I will go over all the fittings for security then bleed again. The last time I bled the brakes they were nice and firm with about 1/2 inch push on the brake pedals. They didn't stay that way and I think there is air entering the system somewhere.
 
Well, I'll take a guess....

Other than "Bernouli's Theorom", I know little about fluid dynamics, but if bubbles are showing up in the system, there MUST be air in the line! Normally, air bubbles will get smaller or disappear when pressure is applied to any sealed fluid system. If bubbles are appearing when you apply pressure, I would think that fluid is being forced into the threads at the tee fittings and forcing the air that normally "hides" there into the line. Did you use teflon tape on the threads? Try that and rebleed the system to see if it helps.
 
Warning !!!

Teflon tape is pretty much a no-no.

The air is either already in the system, from not bleeding all out, or it is entering somewhere.

You mention that the bubbles appear at the top of the nylon lines, near the "T" fitting at the firewall.

Have you looked closely at them, to see if they are moving downward toward the master cylinders, or going up, toward the reservoir????

This will give you a hint as to where they are coming from, so that you can take the correct action.

IIRC, the nylon "T" from Vans uses an "O" ring seal on the lines, finger tight is all that is needed, if installed correctly. Pipe thread into the master cylinder. Locktite makes a formula --545--for pipe thread sealing that will do wonders for you, if the problem is at this location. There are others, but this is what my A/P/tech consular uses.
 
Last edited:
Fred,

The nylon TEE fitting in on the low pressure side of the brake system. There should be very little fluid pressure between the reservoir and the copilot cylinder other than the fluid pushed back up into the reservoir when the brake pedal is released. However, when a brake pedal is pushed, the backside of the pedal piston will create a fairly strong vacuum drawing in fluid from the reservoir. I think it is possible for this vacuum to draw in air through a loose or poorly sealed fitting on the nylon TEE connector. The fitting may be tight enough to prevent fluid leaking out since there is little to no pressure at that point in the system but, the vacuum generated by pressing a brake pedal could be great enough to suck in outside air through the fitting rather than suck fluid in from the reservior. Anyway, that's my theory and I'm sticking to it ;). I'll try tightening up the fitting a bit and see if that solves the problem.
 
One more thought------debris in the bottom of the reservoir????

Block off the drain hole, and the air will come in.

The old "Path of least resistance" thing.
 
647jc said:
...I think it is possible for this vacuum to draw in air through a loose or poorly sealed fitting on the nylon TEE connector. The fitting may be tight enough to prevent fluid leaking out since there is little to no pressure at that point in the system but, the vacuum generated by pressing a brake pedal could be great enough to suck in outside air through the fitting rather than suck fluid in from the reservior. Anyway, that's my theory and I'm sticking to it ;). I'll try tightening up the fitting a bit and see if that solves the problem.
Click to expand...

I'm not at that stage yet so I didn't know it created vaccuum instead of pressure. :eek: Let us know how it works!
 
Air Bubbles in Brake Line

When I first filled the brakes on my aircraft with fluid by pumping fluid up from the master cylinders the brake fluid pushed all the air up and out the reservoir leaving the lines full, no air bubbles.

Subsequent problems with brake squeal and vibration caused me to drain and refill the brakes several times. On all subsequent fillings the brake fluid would flow upwards through the nylon lines and around (yes around) air pockets leaving the system full and air bubbles in the lines.

I even borrowed a pressure pot and tried blasting those bubbles out with a higher speed refill, no luck. I now have bubbles in two aircraft with no effect on brake performance and no longer worry about the bubbles.

My theory, and I am sticking with this, brake fluid (actually hydraulic fluid) is slicker than air, if you don't want to see bubbles in your nylon supply lines use aluminum or steel lines!!
 
George,

Switching out the lines with aluminum or steal would be very impractical, I think I'll just wrap mine with black electrician tape.
 
Brake line air bubbles

I too had problems with air bubbles in the lines, i was able to correct this by removing the line from the cylinder and little bubbles came out that were trapped in the cylinder. No problem since.
 
exact same problem here as described in the first post...
suspect air is entering at the nylon tee under vacuum but no fluid leak...
tried to bleed several times using various methods.

@n647jc: how did you resolve the issue? what kind of tee replacement?

rgds,
bernie
 
mine is like this too.

I pumped from bottom up then from top down then removed lines between cylinders and drained out bubbles that way too. They never go away. I get it so that the lines look good then upon later inspection there are always air bubbles between pilot and co-pilot brakes. I have no leaks. My theory is that when you use the pilot brakes, you create a vacuum between the pilot and co-pilot cylinders. I can have no visible air bubbles then when pressing the pilot brakes I can see a bubble form and stretch in these lines. Kind of like fizz in a soft drink. You have a time spec that grows when vacuum is created. If my theory is correct that means that the co-pilot cylinder does not allow fluid to flow through it when in the neutral position. I guess this would be easy enough to test if I just remove the lines from the co-pilot side, splice them together then see if the problem goes away.
 
I have a few bubbles in the horizontal portion of the lines also, and no longer worry about them. The brakes work well.

L.Adamson --- RV6A
 
I have the same comment as L.Adamson, once I riveted the fuselage foreskin on, the bubbles were out of site and out of mind. I have about 40 hours flying time now and the brakes are working fine, at least from the pilot side and most likely from the copilot side.

I agree with Bernie and think the nylon tee at the resevoir is the problem and that it allows air to enter the system when a vacuum is created by pressing on the brakes. I suspect replacing the nylon tee with a metal unit would correct the problem.
 
My brakes developed air in the lines, glad I didn't have the steel lines or I wouldn't have known. What I did was disconnect the lines at the masters and let it gravety out, when the air came out I quickly stuck the line back on the master and tightened. I did this over a period of 6 weeks. Finally no more air. Just make sure you keep plenty of fluid in the res. and, make sure you put something to catch the fluid or you will make a mess on the floor of the plane. I also think since the lines to the wheels are steel, I would loosen the bleeders and let them gravety out there also, you'll know when there is air because you will get a pause and then a steady stream again. I think trying to push from the bottom at this point is a waste of time and that air doesn't like to go up, even though you think it will. Let it go out the bottom. That's the only way I could get mine out. Do NOT push on the brakes, make sure you let it gravety out.
 
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