bleeding brakes

[tony]

On 7/23/2019 at 2:53 PM, N201MKTurbo said:

I have found this to be true also. I’m not sure the maneuvering does anything, I have always thought it was the altitude. The lower pressure causes the bubbles to grow and migrate easier.

Whatever is going on, flying and pumping seems to finish the job.

I found the same to be true with the flaps when the actuator was replaced.  The only thing that got the air out was to fly it and the service the reservoir.  

[Shadrach]

4 hours ago, Fly Boomer said:

Any idea if this would work using one of the compressed air and venturi vacuum sources?

I have one that I use that I’ve used on auto cooling systems but never on the plane. Worth a shot if you can get it to seal.

[T. Peterson]

You fellows have thoroughly convinced me that when my brake fluid needs to be changed/bled, I will take my airplane to a mechanic!! LOL!!

[ArrowBerry]

Can anyone tell me how much total brake fluid is in the system? We have a leaking brake and I have a new O ring to try and fix it. Assuming that we still have the old style 5606 fluid, is it possible to drain as much as possible of that out of the system before trying to bleed the brakes, and then topping it up with the newer 83282?

[Kesk]

On 7/23/2019 at 10:18 PM, Tom said:

This is misleading.  The military and airlines don't do a provisional bleed, send the aircraft up to bounce around, then finish the job on the ground with a second go.

There are numerous A&P articles and threads on the internet on this subject.   What is straightforward is that, if you have a healthy cylinders, valves, and connections, you cycle air-free fluid from the bottom-up, preferably at a higher rate of speed (pressure pot or pump device) to push/drag air up, such that the air should be evacuated.  By cycle I mean technically overflow the brake system with a catchment system off the reservoir with several quarts going up and through.  A helper jiggling/tapping along the line and connections helps.

This is ordinarily a bit more complicated, requiring a bit of contraption set up, than a "fill up till full, pump 'em a few times, seems to be holding" approach that many an A&P will do the first time touching the brakes.  Often that's all that is needed.  But when it comes back, you'll hear "you know, them brakes, they can be finicky."  My first plane was a high speed homebuilt with a castoring nosewheel.  Zero tolerance for "come backs."

It's a simple hydraulic system, not rocket science.
 

I've bled many aircraft and it is rarely simple. I've bled Pipers by disconnecting the cylinders, inverting them, and actuating them while pressure is pushed through. That's about all you can do and even then they'll just feel okay. I've only ever needed a hand pump on a Cessna. I bleed Diamonds very frequently and it takes a lot of finesse on some.

Any time you install a new brake cylinder it can be a huge pain, shaking, actuating, tapping, depressing the pedal during bleed, you just have to try and try and try until you get enough air out.  Bleeding aircraft brakes can be maddening at times and take hours.

For instance a few quarts for the flap pump worked fine. I'm sure there's air left in it but 5-6 pumps and it works because each builds pressure consecutively. That's good in my book. But brakes are very unforgiving and the cylinders are often just poorly designed or oriented in general aviation aircraft. Even at work sometimes I get bubbles for 2 minutes in Diamonds and sometimes I get bubbles for 15 minutes...

Edited June 17, 2024 by Kesk

[Shadrach]

On 10/9/2023 at 8:29 AM, ArrowBerry said:

Can anyone tell me how much total brake fluid is in the system? We have a leaking brake and I have a new O ring to try and fix it. Assuming that we still have the old style 5606 fluid, is it possible to drain as much as possible of that out of the system before trying to bleed the brakes, and then topping it up with the newer 83282?

Yes. But it is a pain. I had solid brake pedals and solid flaps (4 pumps to hydrolock) in under two hours from start to finish. Thought I had set a personal record for hydraulic system maintenance. Came out to the hanger 10 days later and everything was soft…

Almost all of the old fluid can be removed from the system with little trouble.

1qt of fluid is more than sufficient for filling the system (brakes and flaps). However, you will want to ensure you have a clean catch can when bleeding so that you can recover clean fluid in the event you need to continue bleeding the system. I switched from 5606 to Royco 756 synthetic.

[Kesk]

On 6/17/2024 at 7:50 AM, Shadrach said:

Yes. But it is a pain. I had solid brake pedals and solid flaps (4 pumps to hydrolock) in under two hours from start to finish. Thought I had set a personal record for hydraulic system maintenance. Came out to the hanger 10 days later and everything was soft…

Almost all of the old fluid can be removed from the system with little trouble.

1qt of fluid is more than sufficient for filling the system (brakes and flaps). However, you will want to ensure you have a clean catch can when bleeding so that you can recover clean fluid in the event you need to continue bleeding the system. I switched from 5606 to Royco 756 synthetic.

Was one of the master cylinders leaking? We're trying a vacuum bleed which some users on here recommended since we got barely passable results through traditional methods (pressure bleed bottom-up 20-30 PSI, constant shaking of the cylinders, gallons of fluid, bench bleeding cylinders with caps on to just have the line off for a split second while fluid was pumped from below so minimal air was introduced). Not a big fan of the Paramount cylinders in general, but since it bench bled fine I still don't think it's the seals. If it's getting worse over time it's probably something in the system, but I've only ever had air introduced from bad cylinders, never seen any problems even from very leaky calipers since they're at the bottom of the system.

 

[Shadrach]

15 hours ago, Kesk said:

Was one of the master cylinders leaking? We're trying a vacuum bleed which some users on here recommended since we got barely passable results through traditional methods (pressure bleed bottom-up 20-30 PSI, constant shaking of the cylinders, gallons of fluid, bench bleeding cylinders with caps on to just have the line off for a split second while fluid was pumped from below so minimal air was introduced). Not a big fan of the Paramount cylinders in general, but since it bench bled fine I still don't think it's the seals. If it's getting worse over time it's probably something in the system, but I've only ever had air introduced from bad cylinders, never seen any problems even from very leaky calipers since they're at the bottom of the system.

 

Both brake cylinders were rebuilt prior to this refill. I don’t think it’s the Paramount cylinders so much as how they are oriented as well as the plumbing. There are nearly flat, lateral bends in the plumbing which make bleeding a challenge. 
 

if there’s air in the flap system, I think it is harder to evacuate with the return adjusted for an 8-12 second retraction. The fluid needs to move with some velocity to take air up stream.

[MB65E]

If the thin beveled washer-spring is cracked in the master cylinder the pedal will feel soft. There can be some confusion with orings in the calipers too. 
-Matt. 

[Kesk]

On 6/21/2024 at 9:53 PM, Shadrach said:

Both brake cylinders were rebuilt prior to this refill. I don’t think it’s the Paramount cylinders so much as how they are oriented as well as the plumbing. There are nearly flat, lateral bends in the plumbing which make bleeding a challenge. 
 

if there’s air in the flap system, I think it is harder to evacuate with the return adjusted for an 8-12 second retraction. The fluid needs to move with some velocity to take air up stream.

Yes I agree. If you take a syringe and fill it with water and air you can easily replicate how annoying it is to get air out when horizontal. Air is hard to move when the cylinder is right side up but when upside down the air moves easily since it's lighter (it's different on a piper which has inverted cylinders because unlike a syringe there is a little nook air can get trapped in because the elbow is not directly at the bottom of the cylinder but like an inch up). Horizontal good luck ever getting that air out. I'd love to have a transparent cylinder just to see what happens on the plane. 

[N201MKTurbo]

I built a machine once for fueling missiles. We had to jack those things up all kinds of ways to get all the air out.

[Kesk]

Well, update on the brakes. Some nice people on Facebook informed us the brass valves are a real PITA. We replaced them with some A114s as ours had o-rings and not stat-o-seals. After doing a normal bleed they got okay, not great, but acceptable. Tried all the tricks we could and they pretty much just ended up passable, about what most Pipers feel like in fleets. Shook the hell out of the cylinders while doing slow pumps with the nose jacked up, did the same after putting it on the ground. Definitely got a lot of air out but even a tiny bit can keep it from feeling super firm.

Had no luck with vacuum from the reservoir vent line with the plug closed, not exactly sure so just did a bottom up pressure bleed. Getting the bleeder screw adapter instead of the whole ATS bleeder kit helped. This would be a lot easier with flexible lines I think.

Edited August 6, 2024 by Kesk

[Mooney-Shiner]

I build this monstrosity to pump the brakes from calipers up. Seems like it worked.