Brake bleed in Longmont on M20F

[BageMooney]

Any A&P's in Denver area that can assist with a brake bleed on an M20F?  My current mechanic has run into some issues, so looking for someone with more experience.  Plane is hangared at KLMO.

Cheers!

-Tom

 

[Shadrach]

20 minutes ago, BageMooney said:

Any A&P's in Denver area that can assist with a brake bleed on an M20F?  My current mechanic has run into some issues, so looking for someone with more experience.  Plane is hangared at KLMO.

Cheers!

-Tom

 

What kind of issues?  I just did a drain and flush replacing the 5606 with Royco 782 Synthetic.  I'd estimate it took me a little under two hours total from drain and flush then fill and  bleed the flaps and brakes to completely solid.  It's pretty straight forward.  If he can't get all of the air out, he should consider putting a vacuum pump on the reservoir.  

[Marc_B]

@BageMooney You might reach out to @Vance Harral, he’s based there and might have connections. 
Or reach out to Rocky Mountain Aircraft Services close by; they service quite a few local Mooneys.  http://www.rockymountainaircraftservices.com

[Vance Harral]

Thanks for the point-out, @Marc_B.  PM sent to @BageMooney

[Mark89114]

I am going to provide some advice that isn't particularly useful.

For whatever reason it is a PITA, these people who have flushed their brakes with no problems good for them.  I think as long as you flush and don't get air in you are good.

The problem gets to be when air gets in, I replaced all of the lines as the exterior lines were quite dry and brittle (somebody who sells lines says every 5 years to replace which is BS, anyway) and i figured why not replace all of them and buy another 20 years of trouble free service.  Big mistake.  My mechanic who I now recognize isn't that good couldn't get them bled.  During the purging process we did get a huge pile of orange snotty material....my brakes never really impressed me from the time I bought the airplane.  Because I rebuilt the master cylinders I had access and by changing their position was able to get a lot of additional air out of them.  I think the problem comes in the original design, reservoir to pilots MC to copilots MC and then to calipers.  I think the bubbles just get transferred back and forth and the near horizontal installation design of the MC allows air to stay trapped in top. 

Brakes still aren't 100% right, I think the next step I will try is putting a vacuum on reservoir as Shadrach says.  

I wish I had useful advice and 100% solution for you.

  

[takair]

I almost hesitate to write this, since it is unconventional and requires some care.  At least in the old Mooney, the master cylinder under the floor sits flat and the inlet/outlet also horizontal.  As Mark indicated, the air can get stuck there and difficult to purge.  I’ve tried bleeding right there, but difficult doing that solo, and still opportunity for air to be at the top of the cylinder.  Getting some flow through there may help….but I’ve always ended up with just a little softness.  Here is where it gets unconventional, but worked for me.  If I recall, the forward fitting faces left.  While flying, with plenty of speed and coordinated, perform a relatively steep, climbing, right hand banked turn……..and simultaneously pump the brakes.  This allows the final few bubbles to to find their way to the left forward fitting and to the reservoir.  This is not a surrogate for a full brake bleed….just those final few bubbles.  Results may vary…use care…perform clearing turns and don’t spin…have fun.

[Shadrach]

2 hours ago, Mark89114 said:

I am going to provide some advice that isn't particularly useful.

For whatever reason it is a PITA, these people who have flushed their brakes with no problems good for them.  I think as long as you flush and don't get air in you are good.

The problem gets to be when air gets in, I replaced all of the lines as the exterior lines were quite dry and brittle (somebody who sells lines says every 5 years to replace which is BS, anyway) and i figured why not replace all of them and buy another 20 years of trouble free service.  Big mistake.  My mechanic who I now recognize isn't that good couldn't get them bled.  During the purging process we did get a huge pile of orange snotty material....my brakes never really impressed me from the time I bought the airplane.  Because I rebuilt the master cylinders I had access and by changing their position was able to get a lot of additional air out of them.  I think the problem comes in the original design, reservoir to pilots MC to copilots MC and then to calipers.  I think the bubbles just get transferred back and forth and the near horizontal installation design of the MC allows air to stay trapped in top. 

Brakes still aren't 100% right, I think the next step I will try is putting a vacuum on reservoir as Shadrach says.  

I wish I had useful advice and 100% solution for you.

  

It is a PITA for first timers and sometimes seems to stump seasoned Mechanics as well.  A lot of the time it boils down to not having the right tools for the job, sometimes there are assembly problems with a rebuilt pump (I've run into this twice in the last 6 months).  However, I system that has had no maintenance other than a drain and flush should be fairly easy to return to service.  The reason I suggested the vacuum hand pump is because it will eliminate air from almost all of the system pretty quickly. The exception being the pump cylinder, The last bit of any air trapped in the pump will have to work it's way out. 

I was not bragging, I was merely posting what is possible when one has the right tools and processes for the job.  I was working alone and took the system from full of fluid, to full of air, to full of fluid in under two hours. It is not rocket science, but small details can impede success. 

If the flap pump has been removed and rebuilt, that adds another area of potential problem.  In the last six months I have encountered two different aircraft that have had what seem to be improperly serviced flap pumps (both pumps serviced by MSCs, though I don't recall which) installed.  Full disclosure, I was not on site for either situation. In one case the pump was returned to the MSC for warranty repair. Post warranty reinstallation went flawlessly.  In the other, I sent them a pump that I had on the shelf and they were able to coble together a working pump by "Frankensteining" them together.

 

[Yetti]

The pumping up with an oil can from the caliper bleeder has always worked for me.   You have to have a good oil can with no leaks.  Small piece of tubing and pump rapidly to push the air out.  10 pumps should do it.  20 pumps if it is bad.    Safety wire the tubing to the oil can.  Shorter tubing about 3".  bleed the tubing and don't push air into the system.  Have someone watch the reservoir and you should see the bubbles.  Also a Turkey injector to remove extra oil.  What is an oil can you ask.  https://www.zoro.com/goldenrod-oiler-6-fl-oz-capacity-steel-5-in-spout-length-6-oz-reservoir-capacity-600/i/G9972462/?utm_source=google&utm_medium=surfaces&utm_campaign=shopping feed&utm_content=free google shopping clicks&campaignid=20749401279&productid=G9972462&v=&gad_source=1&gclid=CjwKCAjwkuqvBhAQEiwA65XxQJYdbCLuQpsKINw0q9ZFk-2SOACb9QkjirC5f3TPEPtcbfdq0b4U5xoCIkEQAvD_BwE&gclsrc=aw.ds

[N201MKTurbo]

I have also been using an oil can for years and it always works. I use a piece of clear plastic tubing from the oil can to the caliper. That way you can see if you are pumping in any air and stop before it gets to the caliper. 

[EricJ]

I think some of it has to do with luck as well, whether or not and where any bubbles might get trapped.    I seem to recall only once or twice where it was straightforward for me, including working with my previous IA who always used a pump can at the caliper.

I usually start with a pump can at the caliper, and if that doesn't work then I'll alternate between the pump can and a mighty-vac at the caliper.   Using a clear tube at the caliper you'll be able to see if any bubbles come out, and usually it's a string of tiny bubbles that may be hard to see.     Usually the final iteration that gets it to work will be the pump can. 

YMMV.   There's a local high spot in the system where the soft line meets the hard line in the wing, and the master cylinders make a sort-of local high spot as well.   Those both make bleeding trickier than some systems, so I don't find it surprising that some airplanes are hard to bleed.    Yankees and Cessna singles are super simple in comparison, with the reservoir at the master cylinder being the high spot.    My neighbors with Bonanzas complain enough that I get the impression they're much harder to bleed. 

[Shadrach]

6 hours ago, takair said:

I almost hesitate to write this, since it is unconventional and requires some care.  At least in the old Mooney, the master cylinder under the floor sits flat and the inlet/outlet also horizontal.  As Mark indicated, the air can get stuck there and difficult to purge.  I’ve tried bleeding right there, but difficult doing that solo, and still opportunity for air to be at the top of the cylinder.  Getting some flow through there may help….but I’ve always ended up with just a little softness.  Here is where it gets unconventional, but worked for me.  If I recall, the forward fitting faces left.  While flying, with plenty of speed and coordinated, perform a relatively steep, climbing, right hand banked turn……..and simultaneously pump the brakes.  This allows the final few bubbles to to find their way to the left forward fitting and to the reservoir.  This is not a surrogate for a full brake bleed….just those final few bubbles.  Results may vary…use care…perform clearing turns and don’t spin…have fun.

This last time I did my brakes, I was alone.  Knowing this would be a challenge, I employed the following procedure which worked flawlessly.

Tools:

• Mighty Vac  - This is simply a hand pump that draws air out of the system.  This inexpensive pump from Harbor Freight or any other brake/clutch bleeder would work just as well.
• Motive Power Bleeder This is simply a pressure pot the pushes fluid into the system. Any other pressure bleeder would work as well. I have seen garden sprayers repurposed as pressure pots as well.
• 1/4" AN flare fitting for Hydraulic reservoir.
• Various fittings and several feet of vinyl tubing in different sizes.
• Various hose clamps or zip ties.
• Clean Bucket/Catch can

 

Procedure:

Flaps first -

1. Make sure flap lever is in the up position.
2. Attach 4' of clear vinyl tubing to flare side of 1/4" AN fitting and clamp to seal. This will serve as an overflow.
3. Remove plug from fluid reservoir and and thread on AN fitting with tubing.
4. Ensure tubing outlet is in a bucket/Catch can.
5. Fill pressure pot with one qt of Hydraulic fluid. Royco 782 synthetic is superior to 5606 in my opinion. They can be combined if not fully draining the system.
6. Attach pressure pot to the T fitting on the flap actuator with a clamp. This is the lowest point in the system. It is accessed though a small oval panel just ahead of the stub spar. It is a tight space with which to work. Make sure the pressure pot hose is well secured to the fitting and is not kinked.
7. Pressurize pot (30-35PSI is plenty).
8. Get into the plane and watch for fluid at the reservoir overflow tube. As soon as fluid is visible in the hose, begin pumping the flap handle vigorously. You should feel it pulling fluid into the pump cylinder.
9. Exit plane and have actuator T fitting cover on hand.  
10. Depressurize pot, then quickly remove hose from T fitting  and replace with cover.
11. Clean up hands/remove gloves.
12. Pump flaps do to lock out position and leave them there. Does not matter how many pumps it take at this point
13. Using a straw as a thumb siphon, reduce the fluid reservoir to ~half full
14. Empty catch can into pressure pot
15. move on to the brakes

Brakes -

1. Attach pressure pot to bleed nipple on bottom of caliper and secure with clamp or zip tie.
2. Pump pot to 30-35psi
3. Open bleed nipple 
4. Get in the airplane and pump the appropriate peddle as fluid is pushed into the system. You will feel fluid enter the brake cylinder.
5. When over flow is noted, get out of plane and close the bleed nipple.
6. remove pressure from pot and disconnect from caliper.
7. Repeat process on other brake circuit
8. Using a straw as a thumb siphon, reduce the fluid reservoir to ~ half full
9. Attach hand vacuum to reservoir.  This can be done using the vent on the top of the reservoir or the fill opening; it does not matter which but the other will need to be blocked so it is air tight.
10. Draw the system down to 20- 25inHg and position the pump so that the gauge can be seen from the cockpit
11. Raise deployed flaps while system is under vacuum. Actuate flap handle (lever in up position) and brake peddles from cockpit while system is under vacuum.
12. Release vacuum and test systems.
13. Repeat if needed
14. Remove pump, unblock reservoir vent and go fly.

 

Notes:

Not all hydraulic reservoirs are the same. Depending on year, some are welded aluminum cylinders with AN fittings and some look more like a repurposed solvent container. The person servicing the system will have to procure the correct fittings to work with the installed cylinder.

Either flaps or brakes can be done independently without doing the other. I personally prefer to start with flaps when doing both. 

It may take a few days for all of the air to vacate the flap system. The best way to facilitate the process is to pump the flaps 3/4 down and leave them for a day (or a week).

If a freshly rebuilt flap pump is being installed, fluid will travel more easily through the system with the retraction speed adjustment screw backed out to max flow. This is not necessary, but may make the process easier. Conversely, if the retraction screw is inadvertently tightened all the way down, fluid will not flow through the system.

I welcome comments if it appears I have left anything out.

[Shadrach]

1 hour ago, EricJ said:

I think some of it has to do with luck as well, whether or not and where any bubbles might get trapped.    I seem to recall only once or twice where it was straightforward for me, including working with my previous IA who always used a pump can at the caliper.

I usually start with a pump can at the caliper, and if that doesn't work then I'll alternate between the pump can and a mighty-vac at the caliper.   Using a clear tube at the caliper you'll be able to see if any bubbles come out, and usually it's a string of tiny bubbles that may be hard to see.     Usually the final iteration that gets it to work will be the pump can. 

YMMV.   There's a local high spot in the system where the soft line meets the hard line in the wing, and the master cylinders make a sort-of local high spot as well.   Those both make bleeding trickier than some systems, so I don't find it surprising that some airplanes are hard to bleed.    Yankees and Cessna singles are super simple in comparison, with the reservoir at the master cylinder being the high spot.    My neighbors with Bonanzas complain enough that I get the impression they're much harder to bleed. 

Did you mean Mighty Vac on the reservoir?   I cannot imagine putting a vac at a low point in the system.

[EricJ]

19 minutes ago, Shadrach said:

Did you mean Mighty Vac on the reservoir?   I cannot imagine putting a vac at a low point in the system.

Mighty vac at the caliper.   It draws fluid out the caliper, so it does the same as putting a pressure pot on the reservoir.    On many systems that's the preferrable method to keep from pushing any system debris/dirt/sediment from the low point (caliper) back up into the master cylinder.

[Shadrach]

29 minutes ago, EricJ said:

Mighty vac at the caliper.   It draws fluid out the caliper, so it does the same as putting a pressure pot on the reservoir.    On many systems that's the preferrable method to keep from pushing any system debris/dirt/sediment from the low point (caliper) back up into the master cylinder.

If debris is a concern, the system should be thoroughly flushed.  I have never found debris in a an aircraft brake system. I have however found gelled fluid, and it would seem that it is easier to push it out using gravity to your advantage by pushing it out of the low point fresh fluid. This can be accomplished by actuating the brake cylinders with the bleed valve open.

As for bleeding a flushed system, it seems counterintuitive to try to try to suck the lighter substance down through the heavier substance. In both of your scenarios you’re trying to suck or push the lighter of the two substances in the system, namely air, down and through the heavier substance, namely hydraulic fluid, and then out of the bottom of the caliper…  

A pressure pot placed at the caliper pushes fresh fluid in at the low point driving the air up an out through the reservoir.  A vacuum on the reservoir creates a low pressure area at the high point providing additional encouragement for the air to move up and out and without sucking fluid into the pump. There may be applications where the process you describe is beneficial, but I can’t see how it would be beneficial for bleeding a Mooney (or most any other GA aircraft). The bleeding procedure in the factory MM is poorly written, but as I read it, it recommend pressure pot at the wheel and over flow tube at the reservoir. The mighty vac on the reservoir technique is mine own though I am sure others have come up with similar methods.

[Yetti]

1 hour ago, Shadrach said:

Did you mean Mighty Vac on the reservoir?   I cannot imagine putting a vac at a low point in the system.

I have done a Mighty Vac on the reservoir.  You can get an acceptable level of pedal.   Then Push with the oil can to get the final bubbles out.

[Shadrach]

3 minutes ago, Yetti said:

I have done a Mighty Vac on the reservoir.  You can get an acceptable level of pedal.   Then Push with the oil can to get the final bubbles out.

Mighty Vac was the last part of the process for me. I was able to get an acceptable amount of peddle just from driving fresh fluid from caliper to reservoir. Putting a vac on the reservoir just made it easy to get the remaining air out of the whole system.  The brakes were relatively quick and easy from start to finish.  After this last flush and fill, I came away confident that I had dialed the procedure for one man hydraulic system maintenance.  

[Mark89114]

As I mentioned, my brakes are still not 100% acceptable.  As others have stated here I am going to put a vacuum on the reservoir and let it sit.

We tried pushing fluid from calipers up to reservoir, but the air seems to be stuck and I guess has no incentive to move out of the way.

And if that doesn't work I am going to leave the vacuum on top and apply fluid pressure through the caliper as well.  

[EricJ]

53 minutes ago, Shadrach said:

If debris is a concern, the system should be thoroughly flushed.  I have never found debris in a an aircraft brake system. I have however found gelled fluid, and it would seem that it is easier to push it out using gravity to your advantage by pushing it out of the low point fresh fluid. This can be accomplished by actuating the brake cylinders with the bleed valve open.

You often don't know when debris or dirt is an issue or not.  A deteriorating hose or seal can create debris, etc., and you have no way to know unless/until it gets disassembled.   On race cars we just always did top down for this reason, as getting a chunk of something undesirable in the MC at a bad time can ruin your day.   Next to tires, the brakes are one of the most important systems on a track car, so this was a common practice.   I don't see any reason not to do it on an aircraft system as well when needed.    Another advantage of only pulling fluid in one direction is that you always put fresh fluid in the reservoir and then draw it down, so you know you are always pulling fresh fluid through the system if you only go one way.   Years ago there was a good race brake fluid supplier that sold their fluid in two different colors, blue and amber, and when doing a full bleed/flush you just pulled fluid through until the color changed at the caliper.    On race cars it is a bit more important to get fresh fluid into the caliper as that's where the heat goes, and boiling or overheating the fluid changes its characteristics a bit and you can wind up with a soft pedal even if there's no air.

53 minutes ago, Shadrach said:

As for bleeding a flushed system, it seems counterintuitive to try to try to suck the lighter substance down through the heavier substance. In both of your scenarios you’re trying to suck or push the lighter of the two substances in the system, namely air, down and through the heavier substance, namely hydraulic fluid, and then out of the bottom of the caliper…  

Air bubbles are suspended in the fluid, often as lots of tiny bubbles, and the fluid viscosity keeps them from moving very quickly.   The way to get the bubbles out is just to get the chunk of fluid out that is currently suspending them, and you can do that effectively in either direction.    Bubbles may migrate to the high spot over night or over a long period, but it's rare to chase one during an active bleed.    You can see this with a clear hose, they don't move fast at all.

53 minutes ago, Shadrach said:

A pressure pot placed at the caliper pushes fresh fluid in at the low point driving the air up an out through the reservoir.  A vacuum on the reservoir creates a low pressure area at the high point providing additional encouragement for the air to move up and out and without sucking fluid into the pump. There may be applications where the process you describe is beneficial, but I can’t see how it would be beneficial for bleeding a Mooney (or most any other GA aircraft). The bleeding procedure in the factory MM is poorly written, but as I read it, it recommend pressure pot at the wheel and over flow tube at the reservoir. The mighty vac on the reservoir technique is mine own though I am sure others have come up with similar methods.

All of those methods work, and I usually start with a hand pump at the caliper and push the fluid up, just because it's the most convenient way on the airplane and a good way to keep from introducing air at the caliper.    When that doesn't work, which is about half the time, I just disconnect the pump and connect the mighty-vac and go the other way for a while.   Usually an iteration or two of that gets it done when it's being difficult.

[Shadrach]

34 minutes ago, Mark89114 said:

As I mentioned, my brakes are still not 100% acceptable.  As others have stated here I am going to put a vacuum on the reservoir and let it sit.

We tried pushing fluid from calipers up to reservoir, but the air seems to be stuck and I guess has no incentive to move out of the way.

And if that doesn't work I am going to leave the vacuum on top and apply fluid pressure through the caliper as well.  

I cannot speak to the M20R but I can say that some of the vintage birds seem to have a high spot between the cylinder and the caliper when resting on the gear.  I don't know that this is actually the case, but the sometimes stubborn pockets of air that are challenging to evacuate make it seem so.  Rapidly pushing fluid up from the caliper seems to be the best way to drive it out of the system. 

[skydvrboy]

It's been awhile since I did this on my bird, but we pushed fluid up through the system with a pressure bleeder and got it most of the way.  I thought the brakes were still a little soft so the mechanic recommended I take it up to 10,000' or more to pump the brakes and cycle the flaps a few times.  Upon landing the brakes were noticeably better and good enough that I thought they were acceptable.  As I flew after that for awhile, I'd pump my brakes at altitude and shortly I found them to feel solid, at least for airplane brakes.  They're never going to be as good as the brakes on a sports car, so you have to set your expectations accordingly.  If you've seen the brake pads on each, you'll know why the sports car has so much more braking force.

[takair]

If this diagram is correct, the horizontal master cylinder ports on the R face right.  They also look like flex lines.  So, in theory, one could loosen the master cylinders and rotate to the right while bleeding to get the inlet higher and get the air out.  In my old E, this is not possible, hard lines prevent it.

In my previous post I mentioned pumping the brakes while climbing and banking.  In this case the bank would be to the left. in my single experience doing the climb and bank, the brakes felt notably better in three pumps.  It sounds like skydvrboy also did it while in flight with success….I suspect bank and climb would have helped even more.  Your mileage may vary.

[Z W]

When I tackled this job, I drained the old fluid out through the calipers while keeping the reservoir in the tail full.  Got a fair amount of chunky brown fluid out of the calipers when I opened the bleeders.  Kept going until fresh red 5606 came out.  Still ended up with some air in the system due to changing the old stiff brake lines at the same time.

Refilled from the bottom up using a pressure pot.  Went OK, but the cheap Amazon pressure pot wasn't really up to the task.  Next time I would buy a better one or try the oil can linked here.  And the 5606 has eaten up that plastic pressure pot.  I would use the synthetic fluid if doing it again, and also order the AN fitting to seal up the reservoir in the tail, which I did not have.  Brakes worked afterwards but weren't even, had some softness in the right one until the second pump.

Flying, changing attitudes, and pumping the brakes got it to better, but not perfect.  Don Maxwell's shop fixed it all up at the next annual.  Don't know what they did, but they were better at it than me.  It seems to be a job best done by someone with the proper equipment and fittings, and maybe some experience, if that's an option.

[Shadrach]

17 hours ago, EricJ said:

Air bubbles are suspended in the fluid, often as lots of tiny bubbles, and the fluid viscosity keeps them from moving very quickly.   The way to get the bubbles out is just to get the chunk of fluid out that is currently suspending them, and you can do that effectively in either direction.    Bubbles may migrate to the high spot over night or over a long period, but it's rare to chase one during an active bleed.    You can see this with a clear hose, they don't move fast at all.

I’ve been thinking about this and it does not make sense to me and here is why. I have seen fluids more viscous than hydraulic fluid rapidly degassed befor they cure. A degassing chamber rapidly (necessary given the curing times) is used to remove large and fine bubbles from epoxy resins and silicone. It uses about 1 bar of vacuum to do so. The Epoxy and silicone are significantly more viscous than 5606.
Why is it that you believe that hydraulic fluid cannot be degassed by applying one bar of vacuum to the system at the reservoir? 
 

 

[N201MKTurbo]

So, you have to look at the behavior of bubbles in small tubes. If the bubbles are big enough to go from wall to wall, called a slug of air, they usually won't move through the tube. For the slug to move, the liquid must flow around the slug. that happens very slowly if at all. Bubbles are small spheres of air that are smaller than the diameter of the tube.

Applying vacuum will cause gas that is dissolved in the liquid to come out of solution as bubbles , possibly forming slugs. Vacuum will only cause the slugs to grow in size.

I have found in some previous R&D jobs that applying cyclic vacuum may encourage the slugs to move.

There are a lot of papers written about this, but most are behind paywalls.

[EricJ]

2 hours ago, Shadrach said:

I’ve been thinking about this and it does not make sense to me and here is why. I have seen fluids more viscous than hydraulic fluid rapidly degassed befor they cure. A degassing chamber rapidly (necessary given the curing times) is used to remove large and fine bubbles from epoxy resins and silicone. It uses about 1 bar of vacuum to do so. The Epoxy and silicone are significantly more viscous than 5606.
Why is it that you believe that hydraulic fluid cannot be degassed by applying one bar of vacuum to the system at the reservoir? 
 

 

To add to what Rich just said, you can see this yourself if you use a clear tube to bleed your brakes, which, as previously mentioned, also has the advantage of letting you see and make sure that you're not pumping bubbles into the system.    This is a good reason to let your fresh supply sit for a day or so that if got sloshed around any trapped bubbles have had time to get out.   Anyway, with a clear tube you can see that trapped bubbles don't move very fast if left alone.    You need to get the fluid containing the bubble out, and sometimes that's not trivial in systems with a number of discontinuities and local high spots.   Degassing by drawing the pressure down pulls gas out of solution to the nearest surface, which might be a trapped bubble/slug and just add to its size.

Going both directions like I mentioned before may help dislodge a bubble trapped at a discontinuity like a fitting or in a corner like inside the master cylinder.   At least that's my theory so far from experience.

I've done way too many bleed and flush jobs on cars with complex brake systems including ABS manifolds and pumps, and for some reason they typically bleed easier than some airplanes.   Having the MC so far from the reservoir and not at the highest point in the system probably contributes to that.