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  #1  
Old 08-02-2008, 03:03 AM
Lycosaurus's Avatar
Lycosaurus Lycosaurus is offline
 
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Location: Ottawa, ON
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Question Brake fluids .... my dilemma

I bought the EPDM o-ring kit from Charlie Kuss a while ago, and had upgraded my BUNA N caliper and master cylinder seals with these o-rings, as well as changing out the pilot brake lines with teflon-lined hoses. The intent was to use DOT 4 automotive fluid, since it would mitigate a fire should the aluminum lines or caliper seals rupture at the wheels.

I have since done a little more research, and see that DOT 4 is only used in a sealed system, due to issues with moisture absorption. This fluid is highly hygroscopic, and it is typically recommended that it be flushed out every year or two, when used in a sealed system. Even moist air permeating the pores of the hoses will supposedly degrade the fluid over this time.

In our RVs, the brake system is open to the air, with a breather located at the the top of the firewall forward fluid container. I believe that this fluid would deteriorate very rapidly (weeks), since it is recommended that once opened, the remains of a bottle of fluid be discarded.

Seems that the upgrade to DOT 4 fluid is not for me (unless somebody here can convince me otherwise).


A compromise is the use of mil-h-83282 fluid, which is still flammable, but has a higher flash point than mil-h-5606. As it stands, it would seem that my EPDM seals are compatible with ester fluids (DOT 4, Skydrol), but not with the mineral base (5606) or synthetic hydrocarbon based (83282) fluids.

Here's a link for seal compatibility: http://www.lumaco.com/oringmat.htm

This leave me with the choice of using Skydrol and keep the present EPDM seals (I hear Skydrol is quite a skin irritant, though MSDS downplays this), or change the seals out to Viton (anybody sell a seal kit?) so I can use mil-h-83282. I would prefer not to go back to BUNA N, as it has a low temperature rating.

Skydrol is mildly hygroscopic (has an ester formulation), but also acidic (not sure this is really compatible with our aluminum lines.
Skydrol FAQ: http://www.skydrol.com/pages/faqs.asp

Not sure where to go from here. Leaning towards changing seals to Viton and going the mil-h-83282 route.

Comments?

Edit:

Skydrol seems to be compatible with aluminum.... see http://www.skydrol.com/pages/materials_chart.asp

also from http://www.skydrol.com/pages/faqs.asp
The basestocks of Skydrol hydraulic fluids are blends of phosphate esters, which are mildly hygroscopic. In order to keep moisture in the air from absorbing into the fluid, fluid containers should remain closed when not in use. Vent holes on reservoirs should fitted with dessicant breathers to absorb moisture from the air.
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Last edited by Lycosaurus : 08-02-2008 at 08:00 AM. Reason: Added Skydrol compatibility info
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  #2  
Old 08-02-2008, 07:46 AM
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DanH DanH is offline
 
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<<Comments?>>

Interesting chart. EPDM seals are only good to 300F? For reference, note 225F for Buna (the stock caliper seal) and 400F for Viton.

I think there are two views here. One desires a very fire-resistant fluid, and that approach says go with DOT4. You must change all the seals, master cylinders too. And yes, DOT4 is for sealed systems only, at least if you operate in a humid part of the world. Water in the system is a risk for corrosion pitting. You may or may not get fluid temperature high enough for a steam bubble problem. Add regular drain-n-fill to your maintenance list.

The other approach says the problem is simply keeping the fluid contained within the system in the event of a hot caliper. You make a simple switch; just change the caliper seals to Viton. If the fluid doesn't escape, it doesn't burn. 5606 is fine, 83282 has a higher flash point just in case.

If EPDM seals only gain the user an additional 75 degrees of operating range, I think the DOT4/EPDM choice starts looking less desirable, in particular on an RV-A. And the Viton caliper seal swap is so easy.

However, I notice you live in Canada, and there lies a catch. I had a conversation with a Cleveland engineer a few years back. He felt there was a lot of merit to Viton caliper seals, except in places where it got real cold in the winter....and Cleveland products are used world-wide. Viton may not seal as well at very low temperatures. Yes, I'd love to hear from somebody who tried Viton caliper seals in the Arctic.

So, you choose. It is all a matter of considered compromise. I live and operate where it is hot and humid, so I'll go with Viton and 83282. It may not be your best choice.

Side story; a few weeks ago a friend chose to abort a departure in his A-36 just before liftoff. He got it stopped on the runway. However, both calipers got hot, cooked the Buna O-rings, and started leaking almost immediately. My friend is not an A&P and it was not his home airport, so it turned into an expensive AOG deal....and he got the same components. Kiss your experimental airplane next time you see it.
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  #3  
Old 08-02-2008, 08:05 AM
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Lycosaurus Lycosaurus is offline
 
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Default Viton nicks easily, not for moving parts

[quote=DanH;244399...I'll go with Viton and 83282. It may not be your best choice. [/quote]

Thanks for the background info, Dan.

I had noted that Viton was not recommended for use on a moving seal (don't have the link for that one). Seems it nicks easily and more readily loses its seal with use (probably in cold weather, and with some contamination).

Found that Skydrol is compatible with aluminum (updated my post above with a link), and being mildly hygroscopic, requires just a dessicant at the vent line. Something I certainly can cobble together. Wondering about local availability ... Airlines should make lots of use of this, but wondering if I can get it in less than 45 gallon drums
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  #4  
Old 08-02-2008, 08:15 AM
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rocketbob rocketbob is offline
 
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You don't want to have anything to do with skydrol, it burns your skin on contact. Nasty stuff.

8 years on my RV with Mercon ATF in the brake lines, and have yet to have to top off the resevoir.
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  #5  
Old 08-02-2008, 08:33 AM
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John Clark John Clark is offline
 
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Default "Sealed System"

Although I'm content with good old 5606 in my application, I have a couple of thoughts on how to use automotive brake fluid. The term "sealed system" is based only on the way the reservoir is vented. Go outside and look at the cap on almost any modern (or not so modern) car. There is a vent in the cap to the outside air but there is a rubber bellows to separate the air and the fluid. Seems to me that if you want the isolation in an aircraft installation, you just need to adapt that technology. Find the smallest rubber bit you can and fabricate a reservoir to match.

John Clark
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  #6  
Old 08-02-2008, 08:45 AM
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Rocketbob is right- Stay away from Skydrol!
How about silicone brake fluid? It's non-hygroscopic and won't remove any paint it accidentally gets on. Pedal feel can be slightly mushy, but might be a good choice. I like Bob's idea about ATF, or maybe good old 5606 just to keep it "airplane like".
http://www.skygeek.com/royprod756pe.html
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Last edited by captainron : 08-02-2008 at 09:13 AM.
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  #7  
Old 08-02-2008, 09:44 AM
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RV8RIVETER RV8RIVETER is offline
 
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Obviously your airplane, so what matters to you most of all is what is most important. I have been trying to make the final call myself. I think the best choices are either DOT5.1, 4 or Mobile ATF.

Moisture is not that big of a concern given the real world moisture measurements. You must, again IMHO replace the fluid on a yearly basis to be cautious. That is what I would do anyway as to flush any contaminants, no matter the fluid. I think it is a big plus that DOT fluid does not burn. So even if you have a leak, no fire. This fluid is also in the cockpit again same reason it is a plus, for me.

Reference:
http://www.bobbyarchermotorsports.com/pdf_2848_2.pdf

http://www.xs11.com/tips/maintenance/maint1.shtml

Good luck in your choice.
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  #8  
Old 08-11-2008, 08:07 AM
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Lycosaurus Lycosaurus is offline
 
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Default Buna and Mil-H-83282

I have made my decision, and will convert back to Buna N, and use Mil-H-83282 fluid. If I were to do this over, I would just leave in the original seals, and just use the better 83282 fluid. Can always be topped off with 5606 if in a bind.

At this point, I don't feel comfortable with the DOT brake fluid and open-air vent that entrains moisture. Heat the brakes, and vapor lock may become the issue. Last thing I want is my brakes locking up on a -9A.

May still consider use of Viton on caliper seals, however they are not rated for as low a temperature as the BUNA N, and I live in a fairly cold region. Also, they require more lubrication, and may abrade the cylinder walls. We'll see...

Thank-you all who chimed-in.
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  #9  
Old 08-11-2008, 03:33 PM
N258RE N258RE is offline
 
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so what would the draw back be of using DOT-5 ? other than mushy pedal

I found this but it applies more to auto's than aircraft :

http://www.xs11.com/tips/maintenance/maint1.shtml
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  #10  
Old 08-11-2008, 04:01 PM
Gary Bricker Gary Bricker is offline
 
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Thumbs up Gary Bricker

Let me start by saying I am the Wagner Brake fluid factory Rep for north Texas. I sell a lot of it. The reason the automotive mfgs. use Dot 3 or 4 fluid it its ability to absorb mosture. This cuts down on rust and corroson. If you use Dot 5 which is silicone based the mosture stays seperate and causes problems. The difference between 3, 4, and 5.1 is the boiling point. Most later model cars have aluminum cyl's and we don't sell 1/4th the Master cylinders that we did when they were cast iron. For several years some of the auto mfg's were really pushing flushing after about 30,000 miles. Some have backed off this. When the fluid gets dark change it. When the fluid gets mosture in it the boiling point goes down. With out looking it up, I think 3 boils around 350, 4 around 425 and 5.1 around 500. We sell mostly 3 for cars and 4 for medium duty and off road. Very little 5.1 is sold. Some of the high end imports use 5. (silicone). There is not a large market for 5 so we don't sell it. That is my 2 cents worth on BF.
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