Dangers of old brake hydraulic fluid

[RobertGary1]

At annual I felt my brakes were working well but decided to replace the flex line in the wheel well. While bleeding the system I found a fair amount of jello-ish old hydraulic fluid. I now notice that not only is it far easier to push the plane back into parking now I'm also using less runway. I had been noticing that I'd been getting a bit close to the rising terrain on climb out (I'm based at a small field in the foothills with immediate rising terrain at the ends of the runway). Now I'm clearing it better.

So now I"m thinking about some of the NTSB reports were planes don't get enough alttitude on take off (including a A36 fatality at my home field) and wonder if this is more of a common issue than we realize.

I've got a pressure pot now and plan to bleed them again every 5 years.

-Robert

[Mufflerbearing]

Brake fluid is hygroscopic which means it really loves water and will pull water out from the atmosphere.  Brake fluid needs to be changed regularly not because it is dirty, but because it will find and hold water which will rust out the brake system from the inside out and if it gets hot enough to steam, you will have a spongy pedal. 

 

[RobertGary1]

Does make you wonder why they don’t use DOT brake fluid instead of general hydraulic fluid. I’ve seen cars 20 years old with original dot fluid working fine. 

 

-Robert  

[RLCarter]

8 minutes ago, RobertGary1 said:

Does make you wonder why they don’t use DOT brake fluid instead of general hydraulic fluid. I’ve seen cars 20 years old with original dot fluid working fine. 

DOT 3 & 4 both will absorb water as well, DOT 5 is synthetic and will draw water but it won't mix. No one changes brake fluid as they should until they have too due to failure, most motorcycle manufacturers say to clean/flush and replace every 2 years, I'm sure the auto makers have a recommendation as well. Aircraft and motorcycles have smaller resivoirs than autos, which takes less time for the fluid to absorb as much as it's going too and start causing issues 

[EricJ]

16 minutes ago, RobertGary1 said:

Does make you wonder why they don’t use DOT brake fluid instead of general hydraulic fluid. I’ve seen cars 20 years old with original dot fluid working fine. 

 

-Robert  

It's no better.   Brake fluid absorbs water, too.    We flush brake fluid in race cars reasonably frequently because the absorption of water dramatically reduces the boiling point in most brake fluids, which results in fluid boiling in the calipers and having to pump the brakes up to get any response.   You have to spend a lot to get brake fluid that is immune to that.    Street cars seldom get the brakes hot enough to notice, so it never gets changed, but the fluid probably has a ton of water in it after that long. 

At least in small airplanes the reservoir is usually metal, which helps.   The plastic brake fluid reservoirs in most cars allow moisture through, so the fluid is always exposed to ambient humidity.

 

[Yetti]

19 hours ago, Mufflerbearing said:

Brake fluid is hygroscopic which means it really loves water and will pull water out from the atmosphere.  Brake fluid needs to be changed regularly not because it is dirty, but because it will find and hold water which will rust out the brake system from the inside out and if it gets hot enough to steam, you will have a spongy pedal. 

 

I think you are talking about DOT fluids.   My understanding is MS 5606 Airplane brake fluid is basically mineral oil.   mineral oil is supposed to repel water.   BMW motorcycles use mineral oil for their brake fluids.

[N201MKTurbo]

If you want to use something else, use Dexron ATF, it is essentially the same stuff.

5606 is cheap enough. Changing to a different fluid would require evaluating all the seals and hoses. 5606 works just fine, don’t you have anything else to worry about? 

[EricJ]

I think MIL-PRF-83282D or MIL-H-83282 are approved for Mooneys, which are just the synthetic replacements for 5606.   Commercial options are Royco 782 and Aeroshell 31, iirc.   The last time I looked I think they're fully compatible with 5606 and can even be mixed (miscible), so no issues with seals, etc. 

There's a newer spec, MIL-PRF 87257, which is supposed to be a bit better at low temps, but I don't know that it has been formally approved for Mooney use or where to buy it.

And, yeah, 5606 is cheap and not that hard to flush once in a while.   Anything that's left in there for a decade or more is likely to be a bit dirty and gross.

[RobertGary1]

Is there a certain time period folks like @M20Doc recommend customer flush?

 

-Robert

[N201MKTurbo]

Oh, my slave cylinders start leaking about every 5 years or so. I take them off, hone the bores, put on new o rings and bleed them. When I bleed them it pretty much flushes the system, so .... every 5 years!

[RobertGary1]

17 hours ago, N201MKTurbo said:

Oh, my slave cylinders start leaking about every 5 years or so. I take them off, hone the bores, put on new o rings and bleed them. When I bleed them it pretty much flushes the system, so .... every 5 years!

That’s odd. In 20 years I think I’ve change the seals in one of the cylinders. 

-Robert 

[N201MKTurbo]

1 hour ago, RobertGary1 said:

That’s odd. In 20 years I think I’ve change the seals in one of the cylinders. 

-Robert 

I’m so happy for you, maybe it is the heat and dust here in Arizona. If I bought new cylinders, they would probably last longer, but a few minuets and an o ring every five years works for me, besides, I get new brake fluid....

[RobertGary1]

21 hours ago, N201MKTurbo said:

I’m so happy for you, maybe it is the heat and dust here in Arizona. If I bought new cylinders, they would probably last longer, but a few minuets and an o ring every five years works for me, besides, I get new brake fluid....

I've heard that if you use other MIL spec hydraulic fluid, other than what Mooney calls for, you can also get leaks more often. Not sure if that's the case here or not.

-Robert