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Unbleedable brakes

andrew phillips

andrew phillips

Well Known Member
I have vans standard dual braking system installed as per plans. I read all the threads on bleeding the brakes and decided to do the pump up method. It worked except there were air bubbles around the cylinders. I then bled them conventionally just like a car, making sure to never run the reservoir dry. I thought that got it but apparently not. I saw bubbles in the lines today. I rigged up a reservoir that holds 1 litre of fluid and connected it to the firewall reservoir then I attached a one man bleeding bottle to each brake and pumped a ton of fluid through. I can't get rid of the bubbles at the cylinders. It's almost as if air is leaking into the lines there. There are no fluid leaks anywhere so I think the integrity of the lines is good. Anybody else run into this?
 
First make sure that the lines don't double back anywhere. They must be continuously going up hill. If there is any point where the lines go back down, air will settle at the top of the "loop."
 
How do the pedals feel?

Do the pedals feel fairly firm when you apply brake? While ideally there would be absolutely no air in the system, a tiny amount of air won't be fatal, in my opinion. If the pedals are a bit soft, then you have more work to do. No idea how though

Maybe it is an advantage to have opaque brake lines - that way you can't see any tiny little air bubbles, and thus don't worry about them. Out of sight, out of mind.
 
brake lines do go up and down

The brake lines go vertical (up) from the passenger pedals then horizontal across the rudder pedal assy then vertical (down) to the second set of brakes for the pilot. There is no way to keep everything from going uphill. The air bubbles that are forming are 1/2" to 1" or even longer. They are not the tiny bubbles (which I also have). Surprisingly, the air bubbles do not necessarily sit at the top of the loops.
 
I along with several other builders have also reported this problem. No matter how many times you bleed the system, bubbles continue to appear in the nylon tubes from the reservoir to the copilot pedals and from the copilot pedals to the pilot pedals. My theory is that air is being sucked into the system from the nylon tee fitting at the reservoir. True, this is a low pressure point in the system but when you press a brake pedal down, the master cylinder ‘sucks’ fluid down from the reservoir to replace the fluid it just pushed out to the brake cylinder. I think this suction is strong enough to suck air in through the nylon fitting if it does not seal well. Not sure why I don’t see any brake fluid leak at the fitting other than it being a low pressure point in the system and any fluid being pushed back up into the reservoir would find an easier path to just go back into the reservoir rather than try to escape through a possible poor connection in the nylon tee fitting.

Some builders have replaced the nylon tee fitting with another nylon fitting and have reported the problem solved, other have replaced the fitting with a metal fitting and solved the problem. The bubbles don’t appear to cause any problem (at least for the pilot); I don’t think they migrate past the pilot master cylinders so the pilot brake pedals remain hard. The co-pilot pedal is not quite as hard as the pilot but should be ok for no more than they are used. Another recommended solution to the problem was to replace the nylon lines with aluminum so you couldn’t SEE the bubbles anymore but I think just wrapping the nylon lines with black tape would be an easier solution ;).
 
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From the Bottom up?

Have you tried pumping brake fluid in from the bottom up? A pressure pot, made from a garden sprayer, will pump fluid upwards, and end up coming out of the reservoir, and all the air with it. Use a fitting on the top of the reservoir to attach a tube, and drain the overflowing fluid into a jar. No pumping of the brakes required.:cool:
 
I had (have) the same problem. I've done the garden sprayer technique 4-5 times, each time thinking that finally took care of it, only to have bubbles re-appear. My brakes feel fine to me, so I gave up for now.

erich
 
647jc said:
...My theory is that air is being sucked into the system from the nylon tee fitting at the reservoir. True, this is a low pressure point in the system but when you press a brake pedal down, the master cylinder ?sucks? fluid down from the reservoir to replace the fluid it just pushed out to the brake cylinder. I think this suction is strong enough to suck air in through the nylon fitting if it does not seal well. Not sure why I don?t see any brake fluid leak at the fitting other than it being a low pressure point in the system and any fluid being pushed back up into the reservoir would find an easier path to just go back into the reservoir rather than try to escape through a possible poor connection in the nylon tee fitting..
Click to expand...

We just installed a new brake system in a Three, and used the Aircraft Spruce plastic lines and fittings to run from the reservoir to the brake cylinders. The plastic lines were assembled with great care, but both sides allowed a small amount of air to be sucked in at the line/cylinder junction fittings each time the brakes were actuated and released (without leaking ANY fluid). After multiple bleeding attempts, we replaced the plastic lines with "real" aircraft flex tubing and had no more problems.
 
Bubbles too

I have dual brakes on my 7. I tried about everything but still have a couple of bubbles in the lines between the pilot and passenger side. The brakes work perfect and the bubbles haven't moved, so, I'm calling it good!!!!
 
pull back on pedals?

I have dual brakes and apparently must pull back on pedals while bleeding. I also have some tiny air bubbles visible, but the brakes work/feel fine. C'est la vie.
 
I got rid of the bubbles

I had the same problem in my -4. The brake lines go up from the MC and then down to the wheels. I tried the pump up method over and over but still had a bubble just above the MC. So I installed a tee with a bleeder valve at the top of the loop. I first did the normal pump up method and then pushed the brake pedal with the valve in my tee opened. The bubble went up and out and I closed the valve before releasing the pedal. I sometimes had to repeat once or twice but I was always able to get the air out. That said, I flipped my MCs over and now they bleed perfectly by the pump up method. The inline bleeder is no longer needed but I left them in anyway. One more thing, the automotive method of pumping the brake pedals and releasing the fluid from the wheel cylinder will never work well because the bleeder valve is on the bottom of the cylinder. The air gets trapped at the top of the cylinder. That is why the bleeder valve is on top of an automotive wheel cylinder. I learned that from my racecar.
 
Pumping the fluid in at the wheel cylinders is the only way to bleed aircraft brakes successfully. The pump sprayer (describe above) is convenient but any jug can be used. Bring a fluid line out through a sealed hole in the jug cap and connect it to the bleed fitting on the wheel with a hose clamp. Bleed the air out of the supply line before opening the wheel cylinder bleed fitting. Make another hole in the jug top to insert an air nozzle from your air compressor. (Use a grommet or small rubber hose sealed into the jug cap. The rubber forms a good seal around the air nozzle.) Jiggling the brake pedals helps to get the air out of the brake pedal cylinders.
 
Andrew - Curtis H. of our local EAA245 had significant problems with brake bleeding in his airplane, YLS. As a result he developed a vacuum system for air extraction. According to him it's slow but very thorough in extracting all entrapped air. There are many very useful suggestions in the posts above, some of which may point toward making changes to the aircraft brake plumbing in order to reduce probability of re-introduction of air. If the problem really is just getting the air out for the first time rather than preventing re-introduction of air then I would suggest you give Curtis a call and see if you can borrow his gizmo as it's both local and very effective.
 
Deems Davis said:
Can you provide some more information about the 'real" aircraft flex tubing, where you got it, part #.

THANKS
Click to expand...

Hello Deems,

We used #4 'Stainless Braid Over Teflon' Flexible Tubing with straight #4 Flared Fittings on each end, made up locally (I believe using Aeroquip 666 tubing). Many RV'ers have been happily getting their flexible runs cut to size with proper fittings installed from http://www.bonacoinc.com/ .

I am not familiar with the RV-10 dual-brake line architecture and thus can't comment on the specific connecting fittings required. The method of penetrating the firewall will depend on your specific reservoir configuration. Once inside the cockpit, "Tee" fittings (such as AN824-4D) are commonly employed to split the reservoir line distributions, and AN822-4D fittings typically attach the lines to each brake cylinder.
 
Check out Eastwood Automotive...they sell a vacuum hand pump used to bleed car brakes...cost $35 and works OK but like the rest of you I have had a real problem getting my brakes bled and feeling solid (-4) and have tried every method mentioned.

Glenn Wilkinson
 
I use a 1.99 oil can from the auto parts store. I attached a small length of clear tubing to watch for bubbles. It works great and has bleed the brakes on several RV's.

Also try removing the master cylinder and flipping it over while you bleed.
 
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