What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Brake Line Bubbles

DonFromTX

DonFromTX

Well Known Member
I know this subject has been beat to death previously, I have read everything in the archives, but very little on solutions.
I cannot get my brakes to bleed, there becomes an air bubble in the tubing at the lower master cylinder connection. When fluid is pressured up from the wheel cylinder the bubble disappears into the master cylinder but never comes out the top of the cylinder no matter how much fluid you pump from below. When you stop, the bubble reappears! I have tried tapping the cylinder, pushing on the rudder pedals and such, with no luck. I also tried cracking the fitting to try to let the air out, no luck there either. It seems to defy physics, since the bubble of air SHOULD come out the top of the cylinder.
Others have had the same problem, since when bleeding it appears that the air is gone, only to reappear when you tap on the brake. The bubble is only 1/2 inch long, the pedal feels firm even with the trapped air.
After hours of work, I am totally frustrated with the system. Does anybody have some more ideas to try?
 
Bleeding

The first thought that came to mind for me in reading this was to pull back on the rudder pedal with the master cylinder to be sure it is fully back - that the master cylinder is fully extended. I think they need to be rigged in such a way that they fully extend when released.

(Just in case you have not tried/done this.)
 
Best thing to do is forget about it and enjoy flying the plane. Production planes don't have transparent lines. You bleed them until the pedal is solid and call it a day. As long as you have a firm pedal, all is good.
 
My experience - -

Have had it happen. I found moving the masters back and forth while pressuring fluid from wheel, it seems to move the bubbles along. Do not have to actuate the brake at the peddle, just move the peddle assem back and forth.
 
I saw your post on that John, tried that as well, no success. I am wondering if removing the master cyl and turning it differently so the bubble would come out of the top will work.
I sm getting the feeling that as offered, to just forget about the air and get on with life may be the best. I am betting that LOTS of RV12's have that bubble there if their owners just looked closely.

JBPILOT said:
Have had it happen. I found moving the masters back and forth while pressuring fluid from wheel, it seems to move the bubbles along. Do not have to actuate the brake at the peddle, just move the peddle assem back and forth.
Click to expand...
 
The only way I can bleed my brakes (on an RV-4) is to unhook the master cylinders from the brake pedals and move them around while pumping up fluid from the bottom. By turning this way and that I finally get the last bubble to burp out the top.

Sounds like a huge hassle but one resigned to doing it the time goes fast compared to pumping and cussing as before.

YMMV.
 
You could do it the old fashioned way, and bleed from the bottom instead of pumping from the bottom. It's a pain, but it might get that bubble out of there.
 
That may be the best, however the bleeder on the RV12 is on the BOTTOM of the wheel cylinder, so of course one would have to remove the wheel cylinders and hold the bleeder to the top to remove the air. Not a fun job.
I have had the belief that if one took the master cyls loose and could finally figure out where the air bubble was and allow it to come out the top, that might be the only real way of bleeding them.
JonJay said:
You could do it the old fashioned way, and bleed from the bottom instead of pumping from the bottom. It's a pain, but it might get that bubble out of there.
Click to expand...
 
Don,
I am also having the identical issue. If you find a bleeding method that really works, please let me know. Meanwhile, I'm getting on with life.
Alex
 
Air is not good - -

any air in the line will cause brake loss as some point. It may not affect the pilot side, but passenger side depending upon where the air is. Get rid of it. I think I slightly hand pumped the brakes also once to get the air to move. Also, how much fluid you are pumping makes a difference. Open the bleeder a full turn, and pump from the wheel up.
 
Listen to what Jabarr was telling you. A solid pedal with a 1/2" air bubble will not affect your braking. It is quite normal. Too much air will cause spongy as the air compresses but you will not feel a couple of 1/2" bubbles.
 
In my case, pumping plenty of 5606 up from the wheel is no problem, it is that the bubble will just not go away, it "hides" in the master cyl! Pumping the brake only puts the bubble further from the bottom of the master cyl, then it comes back down when you start pushing fluid back up from the wheel.
 
Don glad to see you working on the plane. Wish I had the answer for you. If I still had a power bleeder we cauld put that on it and move that bubble for sure. Sounds like you have a handle on all the usual tricks. Ted
 
I have become convinced that the construction of the master cylinder is such that the bubble "hides" in a trapped spot higher than the outlet. I don't think any more powerful system would dislodge it. I am going to try to turn the cyl around by disconnecting it, to allow the bubble to move on upstream. That has been used by others with success it seems.
 
I filled my system by hooking a hose from the bleed nipple to the spout of pump type oil can (the kind where you pull a trigger to pump out the oil). Then I put a hose fitting on the reservoir with a clear tube. I kept pumping until I got a clear bubble free column of fluid. I've never ever had any issues with brake operation.
 
This is exactly what I am doing as well, thought the first time I had it well bled too. I am betting that even though the brakes are working fine, if you look carefully at the lower fitting on the master - you will find a bubble. Sufficient pressure, flow, and no bubbles out the top are not a problem. Kinda neat that you can see the flow and watch the bubbles disappear, well most of them at least.:mad:
RFSchaller said:
I filled my system by hooking a hose from the bleed nipple to the spout of pump type oil can (the kind where you pull a trigger to pump out the oil). Then I put a hose fitting on the reservoir with a clear tube. I kept pumping until I got a clear bubble free column of fluid. I've never ever had any issues with brake operation.
Click to expand...
 
jabarr said:
Best thing to do is forget about it and enjoy flying the plane. Production planes don't have transparent lines. You bleed them until the pedal is solid and call it a day. As long as you have a firm pedal, all is good.
Click to expand...

Ditto, same thing here: some bubbles are visible but pedals are firm, good braking, called it a day a year ago.
 
I used a pressure bleeder, but got the same results as the folks that used an oil can. It's not the answer. Like Don, I have opened up the fittings to get the bubbles, but to no avail.
Alex
 
May not be relevant to your application but I had an air bubble trapped in my parking brake valve. I finally unmounted it and manipulated it by hand while the pressure pot did its thing and out it came.
 
We use a 1 gal. Bug sprayer from Ace Hardware. It is a pump up style and cost is around $10. Best money we ever spent. Just screw off the tip and put on a clear polyurethane tube and then insert another poly tube to reduce that size to your caliper nipple size 1/8"). Pour in the brake fluid, pump it up and place it on the caliper nipple. Unscrew the nipple about 1 turn and press the pump handle. The fluid flows so fast through the system you will never have air bubbles.
If you use an oil pump it is so slow that some bubbles can be left in the line. We do this with each caliper then pump the peddles fast and hard for about 10 pumps. Then bleed one more time. You will never have bubbles in your system if done like this. Fluid wasted is about 1-2 ounces and this is from the master cyl. Over flow to make sure all bubbles are gone.

Best $10 you'lol ever spend.
 
Loki - - -

That is what I have, but I also put a 90 degree plastic hose barb fitting in the reservoir and put a clear plastic hose on it, and into a bottle. If you flow more than you need, you have it in the bottle and can put it back in the 'sprayer'. Take a little excess out of the reservoir and ready to go.
 
Oh, and DON - -

That is why I bought a cheap sprayer from the hardware store. Using an oil can, no matter how good it seems, it doesn't work as well. Once I tried for 2 hours, and several tries, but ended up with bubbles. The sprayer solved it. You need a serious flow, and moving the peddles, and a slight hand pumping of each master as the fluid flows. If you saw it work, you'd see how much easier it is. Cheap power bleeder.
 
Last edited:
OK, WalMart aviation department furnished me with this brake bleeder, will try this system now.
2jxvdd.jpg
[/IMG]
 
Well high pressure and high flow rate did nothing at all. The same thing happens, this bubble will "bleed" down into the master cylinder, go into the master cylinder, but never out the top, until you close the bleeder and pump the brake pedal, then it returns to this position.:mad:
I am convinced it is trapped between the piston and the port..
dz76si.jpg
[/IMG]
 
I did the garden sprayer thing - from the bottom/top/ran the cylinders in/out and I still have bubbles. Brakes are firm and work fine. I would hazard a guess that 99% of brake lines in the world have air bubbles in them somewhere.

Move on imho. Get flying. ( I should add that there is an extensive brake break-in and taxi check series of PAP tests before you get airborne.....and also a brake check is on my pre-flight checklist and taxi checklist as well....just be thankful you don't have a BSCU (brake/steering control unit) computer that might decide to turn off your braking and steering at any time...)
 
Last edited:
DonFromTX said:
Well high pressure and high flow rate did nothing at all. The same thing happens, this bubble will "bleed" down into the master cylinder, go into the master cylinder, but never out the top, until you close the bleeder and pump the brake pedal, then it returns to this position.:mad:
I am convinced it is trapped between the piston and the port..
Click to expand...

I have removed stubborn bubbles like this by pressure bleeding while cracking the appropriate compression fitting and allowing the air to escape there. It can be a little messy but if you pivot the rudder pedal assemble fwd most of the fluid will drip off the fitting (catch it in a pan on the floor).
 
My 'POWER' Bleeder - -



Simple, but works. You need the bleeder open at least one turn, pump it up fairly well. Turn on flow, and watch fluid move thru system. As it gets to the first master, move the master back and forth quickly, and slightly pump it until you see no more bubbles. Then stop flow. Go to other side and start it again and see if bubbles in second master. You should see all bubbles move on thru. Maybe you are not getting enough flow. OPEN ONE TURN.
 
Thanks, that was going to be my next attempt as well. Between the pressure on the bleeder pushing it one way, and the master pushing it the other, surely I can find some fitting to crack a bit.
John I have the bleeder open fully, plenty of pressure blasting, but no joy. I ran a full drinking glass thru it this morning, and still have bubbles from both left brake masters. Strangely though the right brakes seem to be completely free of air, very solid pedal too. Could be a sign the left side has become possessed by demons:D
rvbuilder2002 said:
I have removed stubborn bubbles like this by pressure bleeding while cracking the appropriate compression fitting and allowing the air to escape there. It can be a little messy but if you pivot the rudder pedal assemble fwd most of the fluid will drip off the fitting (catch it in a pan on the floor).
Click to expand...
 
DonFromTX said:
Thanks, that was going to be my next attempt as well. Between the pressure on the bleeder pushing it one way, and the master pushing it the other, surely I can find some fitting to crack a bit.
John I have the bleeder open fully, plenty of pressure blasting, but no joy. I ran a full drinking glass thru it this morning, and still have bubbles from both left brake masters. Strangely though the right brakes seem to be completely free of air, very solid pedal too. Could be a sign the left side has become possessed by demons:D
Click to expand...

Part of my problem as well is that when you pour the fluid into the sprayer you should make sure there are no bubbles and also cut a small amount off the bottom of the sprayer to ensure it is pulling in straight fluid. Getting bubbles out doesn't help if you are drawing more in. Like I said - I got most out, have extremely firm brakes, test them every flight, and am confident they work fine - although I try not to put my self in situations where I need to use the brakes that much anyway.
 
Don, I wonder if you are sucking in air somewhere------even as you are purging it elsewhere??
 
Don,
Like I said in post #19, I have tried pressure bleeding and cracking all the fittings. No joy! Small bubbles persist, but the brakes feel fine. I have given up and moved on. Suggest you do the same if the brakes are not spungy.
Alex
 
I painted my interior in grey primer. I inadvertently painted about 1" of a brake line. It took several bleeding attempts before "the bubble" was recognized as a painted portion and I was on a fool's errand!:eek:
 
My right side feels VERY firm and proper. The bubble on the left side makes it feel spongy. I just cannot think I could ever get used to hard brakes on one side and spongy on the other. In looking closer today, there are in fact TWO bubbles on the left system. Both hide in the master cyl until you pump the brakes..
alexe said:
Don,
Like I said in post #19, I have tried pressure bleeding and cracking all the fittings. No joy! Small bubbles persist, but the brakes feel fine. I have given up and moved on. Suggest you do the same if the brakes are not spungy.
Alex
Click to expand...
 
You need to get the air out - -

Brake bleeding
From Wikipedia, the free encyclopedia
Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the brake fluid) are purged of any air bubbles. This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system. The same methods used for bleeding are also used for purging, where the old fluid is replaced with new fluid, which is necessary maintenance.

----------------------------------------------------------------------------------
Air in one side would make it nearly impossible to stop properly. Are you draining the side with air in it before trying again ?
 
No John, that is one trick I had not even considered. Trying to visualize what that could gain me.
JBPILOT said:
Brake bleeding
From Wikipedia, the free encyclopedia
Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the brake fluid) are purged of any air bubbles. This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system. The same methods used for bleeding are also used for purging, where the old fluid is replaced with new fluid, which is necessary maintenance.

----------------------------------------------------------------------------------
Air in one side would make it nearly impossible to stop properly. Are you draining the side with air in it before trying again ?
Click to expand...
 
Don - -

with your 'power bleeder' pumped up to good pressure, if you drain the side giving you trouble, open the bleeder at least one full turn, you should be able to watch the flow move along fairly quickly. As it reached the first master, move it back and forth ( jiggle it ), then use slight hand pumps of the master until you see no more air coming out of it. Then move on to second master and do the same. Have you checked the bleeder to see the hole is clear and fully open ? Note again - you should be able to watch the fluid moving thru the line at a fairly decent rate. It should not be just creeping along.
 
Old School method ?

Back in the 60's & 70's, we bleed brakes as a 2 man job. Have the "outside guy" open the bleed valve when the "inside guy" stepped on the brake pedal. The outside guy would then close the valve before the inside guy let up on the pedal. Repeat until the outside guy got a solid stream of fluid and the inside guy had a solid pedal when the valve was closed
 
Rick - -

you are correct. On autos it works fairly well. The bleeder at the RV-12 caliper is not set up the same way. The bleeder is on the bottom. Pushing the fluid up from the bottom is better in this case because of the bleeder location.
 
Very easy to do, and costs nothing, I have no other tricks up my sleeve at the moment, so will try it for sure.

JBPILOT said:
with your 'power bleeder' pumped up to good pressure, if you drain the side giving you trouble, open the bleeder at least one full turn, you should be able to watch the flow move along fairly quickly. As it reached the first master, move it back and forth ( jiggle it ), then use slight hand pumps of the master until you see no more air coming out of it. Then move on to second master and do the same. Have you checked the bleeder to see the hole is clear and fully open ? Note again - you should be able to watch the fluid moving thru the line at a fairly decent rate. It should not be just creeping along.
Click to expand...
 
You may want to check your parking brake valve to make sure it is fully open. Otherwise you may not get adequate fluid flow from the caliper side to the master to purge the air. My pressure bleeder worked a lot better with valve fully open. Not sure if there is any air in the lines as I changed to SS hoses, but both sides have the same firmness.

John Salak
RV-12 N896HS
 
Earlier the parking brake was partially closed. I found fully open by moving it with pressure below, it is open fully now.
12vaitor said:
You may want to check your parking brake valve to make sure it is fully open. Otherwise you may not get adequate fluid flow from the caliper side to the master to purge the air. My pressure bleeder worked a lot better with valve fully open. Not sure if there is any air in the lines as I changed to SS hoses, but both sides have the same firmness.

John Salak
RV-12 N896HS
Click to expand...
 
I really like the idea of pressure bleeding the system. I would like to make an overflow collection container by temporarily adding a return line to the reservoir. It's just a matter of adding a fitting to a cap that would screw on the top of the reservoir. Can anyone provide the size, and thread of that cap? With that, and a good flow from the bottom up, I am hopeful of eliminating all of the air bubbles.

Tom
 
You must log in or register to reply here.
Back
Top