Bleeding Mooney Brakes

[PT20J]

The left master cylinder on my 1994 M20J was seeping a bit of fluid. I removed both cylinders and had my IA (Scott) inspect them and install new O-rings. I also replaced the hoses to both brake calipers and a hose between the left master cylinder and the parking brake valve, and installed new brake linings. When reassembling, I used a pump oil can to fill the lines and master cylinders with brake fluid.

Now it was with some trepidation that I faced bleeding the brakes because I've never done it before and there are multiple threads on MS about how difficult this is. It turned out to be a nonevent. Scott suggested opening the bleeders and allowing the fluid to gravity drip. I ran a quart of fluid through the system this way to thoroughly flush it. Scott recommended stopping several times and closing the bleeders and slowly moving the pedals through complete strokes several times. He cautioned to move the pedals slowly to avoid aerating the fluid. After doing this we let it sit for a day and then did the standard bottoms up bleeding. Because the feed line from the reservoir to the master cylinders is clear, it's easy to see when all the bubbles are out. It worked great. Brakes are nice and solid.

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[PT20J]

Also, Mooney used Wraplock band clamps in two places on each main gear to secure the flexible brake line to the landing gear leg. This product is obsolete and requires special tools to install as do currently available Band-it clamps. I just used standard hose clamps. I put a few turns of friction tape around the hose at each location to cushion it which appears to be what was done on the original installation.

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[Shadrach]

Glad it worked out! I have had to work harder than that to purge all off the air from my brake system, but the vintage mid bodies have different plumbing. My brake cylinders are due for rebuild this year so we’ll see how it goes.
Did you ask Scott how piston speed “aerates” the fluid? It reads like he thinks hydraulic fluid will cavitate.

[PT20J]

42 minutes ago, Shadrach said:

Glad it worked out! I have had to work harder than that to purge all off the air from my brake system, but the vintage mid bodies have different plumbing. My brake cylinders are due for rebuild this year so we’ll see how it goes.
Did you ask Scott how piston speed “aerates” the fluid? It reads like he thinks hydraulic fluid will cavitate.

I think that was the idea: Moving the pistons rapidly mixes any air with the fluid and he said it will take a long time for it to separate. He also told me that the easiest to bleed are Cessnas, and the worst are Cherokees (four master cylinders and a brake cylinder with plumbing running all over the place).

[MikeOH]

I don't believe cavitation is air bubbles but the vapor form of the fluid when the local pressure drops below the fluid's vapor pressure.  In other words, if air isn't already in there, high speed is not going to create air bubbles. And, as soon as the motion stops the vapor condenses back to fluid.  Also, I suspect the velocity would need to be much higher than can be generated by hand.

Waiting to be corrected...

[PT20J]

11 minutes ago, MikeOH said:

I don't believe cavitation is air bubbles but the vapor form of the fluid when the local pressure drops below the fluid's vapor pressure.  In other words, if air isn't already in there, high speed is not going to create air bubbles. And, as soon as the motion stops the vapor condenses back to fluid.  Also, I suspect the velocity would need to be much higher than can be generated by hand.

Waiting to be corrected...

You may well be right. It's just what Scott told me based on his experience.

[Shadrach]

56 minutes ago, MikeOH said:

I don't believe cavitation is air bubbles but the vapor form of the fluid when the local pressure drops below the fluid's vapor pressure.  In other words, if air isn't already in there, high speed is not going to create air bubbles. And, as soon as the motion stops the vapor condenses back to fluid.  Also, I suspect the velocity would need to be much higher than can be generated by hand.

Waiting to be corrected...

Cavitation is simply a localized phase change from liquid to gas brought about by a rapid drop in static pressure to below the liquid’s vapor pressure…ie boiling.  Cavitation can occur in hydraulic fluid. However, I’m not sure how a brake piston in a cylinder would cause a rapid pressure drop. As you said, If there is existing air in the cylinder, it may be diffused into the fluid by rapid cylinder movement, however, even that seems unlikely. I was just curious about the mechanic’s instruction. Apologies for the thread creep.

[MikeOH]

47 minutes ago, PT20J said:

You may well be right. It's just what Scott told me based on his experience.

My guess, and that's all it is, would be that if there is already some air in the system/cylinder then rapid motion will cause, say a single air bubble, to break into many smaller bubbles that my persist and cling to interior surfaces and therefore make bleeding much more difficult.

[A64Pilot]

Pumping fast won’t of course cavitate the fluid, but it will take air bubbles that are already there and them into foam and the foam being many tiny bubbles is harder to get out. Even on cars don’t pump the pedal fast when bleeding or you will foam the fluid.

Of course time breaks up any foam back into a large bubble that’s easier to remove, so if you make that mistake, just take a lunch break or something.

Done correctly you don’t ever need to even get in the cockpit of a Mooney except to test after the job is done.

Last Summer I re-built my Master cylinders ( I’m an IA) and I was worried about bleeding. I used a bleeder ball, essentially an insect sprayer with hyd fluid resistant rubber bits and flushed most of a quart through each side quickly to push any bubbles up and out.

I was worried having read all the threads, but complete non event, hard pedals right off.

I’m convinced of two things.

1. bottom up is the way to go as of course air bubbles will rise naturally in the fluid column.

2. Push a significant amount of fluid quickly through the system and both the flow rate and the volume will get ALL the bubbles, and you really want to get a good flush if your using the old antiquated fluid that burns easily and oxidizes and turns into goo over time.

Using an oil can works on most aircraft like Cessna’s, but I think aircraft with long complex systems like our Mooney’s really benefit with a pressurized fluid source that can quickly and continuously flush a significant amount of fluid through the system, you only have to buy the tool once.

This is the one I use, I’m sure there are others. I’ve seen some excellent ones made from empty freon tanks for example. The Freon tank one was probably better 

https://www.aircraftspruce.com/catalog/lgpages/hydraulicBleedertank.php?clickkey=4306

[PT20J]

The supply line from the master cylinders to the reservoir in the tail is clear plastic in my airplane. This made it easy to see the bubbles. After the gravity draining, there were very few large slugs of air. It was mainly bubbles. Toward the end the bubbles were tiny and fully entrained in the fluid. We used a pump oil can and piece of clear tubing to attach to the bleeder. We only had to pump about two cups through each side. I used a turkey baster to bring the level of the fluid back down to the full line on the reservoir.

[Shadrach]

@PT20J Do you think it would be possible to pull all of the air out of the system at the reservoir using a hand held vacuum pump like say a mighty vac?

[PT20J]

8 hours ago, Shadrach said:

@PT20J Do you think it would be possible to pull all of the air out of the system at the reservoir using a hand held vacuum pump like say a mighty vac?

As I understand it, the theory for bleeding by pressurizing the system from the lowest point is that the air, being lighter than the fluid, will tend to rise anyway, so you are forcing it the way it tends to go. I guess you might accomplish the same thing by lowering the pressure at the receiving end. Either way, you need to move fluid through the plumbing until all the air is removed.

[Shadrach]

9 hours ago, PT20J said:

As I understand it, the theory for bleeding by pressurizing the system from the lowest point is that the air, being lighter than the fluid, will tend to rise anyway, so you are forcing it the way it tends to go. I guess you might accomplish the same thing by lowering the pressure at the receiving end. Either way, you need to move fluid through the plumbing until all the air is removed.

I always fill from the bottom up, but I suspect the plumbing on the right brake of my aircraft has some high spots where bubbles tend to remain. That side is always more work to fully bleed. I asked about the mighty vac because of my experience with cooling systems in mid-engined cars. Some can be a real PITA.  You can spend a lot of time burping and bleeding or you can use compressed air through a venturi vacuum at the reservoir to draw the system down to 25inHg. This does two things, it verifies he system is air tight and pulls almost all the air out of the system.

[EricJ]

2 hours ago, Shadrach said:

I always fill from the bottom up, but I suspect the plumbing on the right brake of my aircraft has some high spots where bubbles tend to remain. Its always more work to fully bleed. I asked about the mighty vac because of my experience with cooling systems in mid-engined cars. Some can be a real PITA.  You can spend a lot of time burping and bleeding or you can use compressed air through a venturi vacuum at the reservoir and draw the system down to 25inHg. This does two things, it verifies he system is air tight and pulls almost all the air out of the system.

I've had success both ways, and sometimes only by doing it both ways.   The last few times I've had to bleed the system I was only able to get it done by going both directions, some down using a mighty vac, some up using a pump (so always connected at the caliper).    When I'm drawing it down through the caliper I use a mighty vac, and it'd probably work at the reservoir, too, if you had a way to put a sealed up lid on it.    One potential downside to drawing a vacuum at the reservoir is that it may create a larger pressure drop across the length of the supply hose than it usually experiences, and if there is a small leak somewhere along that line it'll draw air into the supply line.   It's why I don't like to use a pressure pot at the reservoir, but lots of people do that, too.

 

 

[Z W]

I got the job done using a cheap plastic pressure pot ($50.00 or so) from Amazon and bleeding from the ground up.  Brakes were a little soft at first but it worked itself out after a flight.  

Fought with the pot quite a bit and next time will buy the $100.00+ version from Aircraft Spruce instead. 

Go for an all-metal tank pot - the brake fluid the Mooneys use seems to soften most plastics.  The pressure pot got a little melted and sticky after sitting in the shop with residual fluid in it.  Also used some large syringes to add and drain fluid from the reservoir in the tail and those quickly turned to mush.  Anything plastic is probably a one-time use for this job.

Also recommend draining all the old fluid out if the job hasn't been done in a while.  Did not do that at first and couldn't get the right side to take fluid when we tried to pump it back up, even though it was dripping fluid like it was open.  Had to pull the bleeder all the way out and step on the brakes, which shoved a bunch of clumpy old goo out and got it flowing freely.  New fluid was a much brighter red color than the old stuff that came out.  And the brakes were working fine before this...

Overall the job wasn't too bad and new brake hoses were long overdue.  Good luck.

[EricJ]

5 hours ago, Z W said:

I got the job done using a cheap plastic pressure pot ($50.00 or so) from Amazon and bleeding from the ground up.  Brakes were a little soft at first but it worked itself out after a flight.  

Fought with the pot quite a bit and next time will buy the $100.00+ version from Aircraft Spruce instead. 

Go for an all-metal tank pot - the brake fluid the Mooneys use seems to soften most plastics.  The pressure pot got a little melted and sticky after sitting in the shop with residual fluid in it.  Also used some large syringes to add and drain fluid from the reservoir in the tail and those quickly turned to mush.  Anything plastic is probably a one-time use for this job.

Also recommend draining all the old fluid out if the job hasn't been done in a while.  Did not do that at first and couldn't get the right side to take fluid when we tried to pump it back up, even though it was dripping fluid like it was open.  Had to pull the bleeder all the way out and step on the brakes, which shoved a bunch of clumpy old goo out and got it flowing freely.  New fluid was a much brighter red color than the old stuff that came out.  And the brakes were working fine before this...

Overall the job wasn't too bad and new brake hoses were long overdue.  Good luck.

If you ever wind up flushing it again you can switch to synthetic fluid, like Royco 782 or Aeroshell 31.    They won't get gross and gooey like the 5606 does.

[OR75]

tapping the brakes while in altitude / lower pressure helps remove bubbles  

[Mark89114]

I am struggling with this as well....my mechanic is not super helpful.

I rebuilt the master cylinders with new o-rings, there isn't a lot to go wrong on this....in my opinion.

Put all new hoses as I was having evidence of a hose coming apart somewhere.  We can't get them bled, the mechanic is convinced we are pulling air....which I think is BS as we would also be showing signs of leaking.  Anyway bought the new seals, not a lot of $, but it irritates me.  My other mechanic says that is nonsense but he isn't doing the annual so I have too much invested to jump ship.  Curious how much slop people have at the end of the Parker 10-24D master cylinders....in my opinion it seems like a lot but there isn't any standard that I am aware of. 

We have tried pressurizing from the top, bottom, etc.  No luck. 

Per somebody elses reccomendation above and the other mechanics, sometimes flying it around allows the bubbles to work their way out.    

[PT20J]

What worked for me was taking a pump oil can full of fluid and filling all the lines and master cylinders as much as possible before tightening the connections (messy). Then I opened the bleeders and let it drip until I ran about a quart of fluid through the system. Then we let it sit for a day before bleeding it bottoms up. My IA claims that most of the problems come from rushing it and that letting it drain and sit for a while helps. I don't know if it does or not, but it worked for me. He also told me that Cherokees are the worst (5 master cylinders including parking brake with lines running up and down and all around); it once took him two full days to bleed a Cherokee. So, at least something is worse. 

[EricJ]

4 minutes ago, PT20J said:

What worked for me was taking a pump oil can full of fluid and filling all the lines and master cylinders as much as possible before tightening the connections (messy). Then I opened the bleeders and let it drip until I ran about a quart of fluid through the system. Then we let it sit for a day before bleeding it bottoms up. My IA claims that most of the problems come from rushing it and that letting it drain and sit for a while helps. I don't know if it does or not, but it worked for me. He also told me that Cherokees are the worst (5 master cylinders including parking brake with lines running up and down and all around); it once took him two full days to bleed a Cherokee. So, at least something is worse. 

My hangar neighbor complains about bleeding his Bonanza brakes and gives me the impression they're the worst thing ever.      

[N201MKTurbo]

I replaced the brake lines on my motorcycle. It took forever to get it right. I don’t think it is the vehicle the brakes are on, it is just brakes in general.

Sometimes it goes smooth as silk, sometimes not so much….

[Kelpro999]

3 hours ago, Mark89114 said:

sometimes flying it around allows the bubbles to work their way out.    

Only if you don’t need brakes for first departure or landing 

[skykrawler]

On 11/2/2023 at 1:29 PM, PT20J said:

What worked for me was taking a pump oil can full of fluid and filling all the lines and master cylinders as much as possible before tightening the connections (messy). Then I opened the bleeders and let it drip until I ran about a quart of fluid through the system. Then we let it sit for a day before bleeding it bottoms up. My IA claims that most of the problems come from rushing it and that letting it drain and sit for a while helps. I don't know if it does or not, but it worked for me. He also told me that Cherokees are the worst (5 master cylinders including parking brake with lines running up and down and all around); it once took him two full days to bleed a Cherokee. So, at least something is worse. 

This ^^^^^......I replaced hose from the hard lines to the calipers on my J (re-using the host ends).    Used the pressure pot to fill the line from the caliper before connecting to the hard line.  No problems at all.

Tossed the straps that held the hose to the gear, wrapped the hose with tape and used tie wraps.

[Mark89114]

As previously mentioned, I am having a heck of a time getting them bled.  My mechanic insisted on doing the same thing over and over.....that is another discussion.

I think the problem is that the dual brake systems during traditional bleeding simply passes the air bubbles from side to side.  What eventually worked is forcing fluid from the caliper up and out from the reservoir, I removed the master cylinders and allowed the air to flow upward, just doing that I could see the bubbles move up and out, shook them around a bit as well and that made some difference.   

[EricJ]

33 minutes ago, Mark89114 said:

As previously mentioned, I am having a heck of a time getting them bled.  My mechanic insisted on doing the same thing over and over.....that is another discussion.

I think the problem is that the dual brake systems during traditional bleeding simply passes the air bubbles from side to side.  What eventually worked is forcing fluid from the caliper up and out from the reservoir, I removed the master cylinders and allowed the air to flow upward, just doing that I could see the bubbles move up and out, shook them around a bit as well and that made some difference.   

On race cars we called that "bench bleeding" the master cylinder, but in cars the MC is often the highest point in the system.