Brake bleed in Longmont on M20F

[Shadrach]

2 hours ago, N201MKTurbo said:

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.

 

26 minutes ago, EricJ said:

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.

Thanks for the explanation gents. I look forward to reading more and doing my own experiments. Should be easy to cobble together some clear tubing to observe the behavior of air in tubing.

The reason I employed the tactic in the first place is due in part to my practical experiences bleeding the cooling systems of water cooled rear and mid-engines Porsches. Under those scenarios the reservoir is the highest point, the water pump and coolant passages through the block and heads are clustered near the low point as are the combination of soft and hard pipes traveling all the way to the front bumper through two or three upright radiators (depending on model) with coolant passages much higher than the rest of the plumbing.  Using a vacuum to fill and evacuate air from those systems is best practice. indeed Porsche sells a $100 coolant system vac rebadged with their emblem for $500. Trying to fill and bleed without a vacuum system can be a challenge that sometime necessitates jacking the ass end and burping the system by reving the engine. 
 

Perhaps the best method for the Mooney is to put a vacuum on the reservoir with the system empty and draw fluid individually at each low point until it reaches the reservoir.

 

[EricJ]

16 minutes ago, Shadrach said:

 

Thanks for the explanation gents. I look forward to reading more and doing my own experiments. Should be easy to cobble together some clear tubing to observe the behavior of air in tubing.

The reason I employed the tactic in the first place is due in part to my practical experiences bleeding the cooling systems of water cooled rear and mid-engines Porsches. Under those scenarios the reservoir is the highest point, the water pump and coolant passages through the block and heads are clustered near the low point as are the combination of soft and hard pipes traveling all the way to the front bumper through two or three upright radiators (depending on model) with coolant passages much higher than the rest of the plumbing.  Using a vacuum to fill and evacuate air from those systems is best practice. indeed Porsche sells a $100 coolant system vac rebadged with their emblem for $500. Trying to fill and bleed without a vacuum system can be a challenge that sometime necessitates jacking the ass end and burping the system by reving the engine. 
 

Perhaps the best method for the Mooney is to put a vacuum on the reservoir with the system empty and draw fluid individually at each low point until it reaches the reservoir.

 

Absolutely familiar with the struggles of getting air out of engine blocks that are a local high point, higher than the rad, etc.

Drawing a vacuum during fill reduces the amount of air that can get trapped, so that's a workable tactic on filling coolant systems.   Watter wetter or something similar can be used to reduce surface tension to help keep the bubbles smaller to purge more easily.   With small tubing the surface tension and capillary action make everything behave a bit differently, though.

[Shadrach]

23 minutes ago, EricJ said:

Absolutely familiar with the struggles of getting air out of engine blocks that are a local high point, higher than the rad, etc.

Drawing a vacuum during fill reduces the amount of air that can get trapped, so that's a workable tactic on filling coolant systems.   Watter wetter or something similar can be used to reduce surface tension to help keep the bubbles smaller to purge more easily.   With small tubing the surface tension and capillary action make everything behave a bit differently, though.

So then, my sub two hour fill and bleed success was not affected at all by putting a vacuum the reservoir...  That's actually good news. Given that I likely spent 30 mins screwing around with different hoses, fittings, zip ties and clamps to adapt the MightyVac to the reservoir, it was actually took just over an hour to do only the stuff that mattered. I guess it is better to be lucky than smart...

[phxcobraz]

Pulling this topic back up. My right brake became useless this past year, though good pedal pressure. Did new soft lines a year ago, got pressure, and it slowly declined. @N201MKTurbo and I found the reservoir dry a while back and topped it off. Never saw any fluid on the ground. Been trying to get it working again. We tried a few ways at annual 2 weeks ago, most recently today I did as stated in here and backbled via a pump oiler at the caliper until I had fluid coming out the reservoir. Then used a vacuum pump and pulled from the caliper.  Did this a few times and always got bubbles out. Never developed any pedal pressure after this. Flaps and other brake work fine. 
 

Perhaps I need some better fittings, should the pressure in the vacuum maintain or decrease as it pulls air?

Alternatively I had someone pump the brake during both push and suck phases to no luck. 

[EricJ]

27 minutes ago, phxcobraz said:

Pulling this topic back up. My right brake became useless this past year, though good pedal pressure. Did new soft lines a year ago, got pressure, and it slowly declined. @N201MKTurbo and I found the reservoir dry a while back and topped it off. Never saw any fluid on the ground. Been trying to get it working again. We tried a few ways at annual 2 weeks ago, most recently today I did as stated in here and backbled via a pump oiler at the caliper until I had fluid coming out the reservoir. Then used a vacuum pump and pulled from the caliper.  Did this a few times and always got bubbles out. Never developed any pedal pressure after this. Flaps and other brake work fine. 
 

Perhaps I need some better fittings, should the pressure in the vacuum maintain or decrease as it pulls air?

Alternatively I had someone pump the brake during both push and suck phases to no luck. 

Can you pump up pressure with the pedal or can you not get any pedal pressure at all?    If it can be pumped up, there's still air in there somewhere, if it won't pump up at all, you may need an MC rebuild.

Sometimes leaving it sit for a while and trying again later lets the air move somewhere else that might be easier to get out.

[phxcobraz]

2 minutes ago, EricJ said:

Can you pump up pressure with the pedal or can you not get any pedal pressure at all?    If it can be pumped up, there's still air in there somewhere, if it won't pump up at all, you may need an MC rebuild.

Sometimes leaving it sit for a while and trying again later lets the air move somewhere else that might be easier to get out.

When I started it the pressure was good, but no brake function on that side. 
Now I have no brake and soft pedal, however the first time I did it the pedal did come back after some pumps. Wouldn’t be surprised if a MC and/or caliper rebuild would be needed. 

[N201MKTurbo]

27 minutes ago, phxcobraz said:

When I started it the pressure was good, but no brake function on that side. 
Now I have no brake and soft pedal, however the first time I did it the pedal did come back after some pumps. Wouldn’t be surprised if a MC and/or caliper rebuild would be needed. 

You should bring it over some time.

[phxcobraz]

6 minutes ago, N201MKTurbo said:

You should bring it over some time.

Haha I was just thinking that. I’ll txt you. 
Gotta say we got timing on point at annual and it’s flying real good!

[N201MKTurbo]

1 hour ago, phxcobraz said:

Haha I was just thinking that. I’ll txt you. 
Gotta say we got timing on point at annual and it’s flying real good!

Tomorrow and next weekend, I’m busy. I could do some evening this week.

[phxcobraz]

An update on my brake issue. Swapping in a rebuilt replacement master cylinder did the trick. The old one was not liking the rebuild, kept leaking.