Vanremog(at)aol.com
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 Posted: Wed Dec 26, 2007 7:37 pm Post subject: Upgraded Brake Fluid and O-rings |
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It's funny how various issues get interwoven even if they're only slightly related (in this case to aircraft brakes), but just to clarify the issues here with regard to both the MIL-PRF-83282 high temp aircraft hydraulic (brake) fluid and the Viton O-rings.
Naturally the O-rings in any hydraulic system (in a brake system that means all of them from the master cylinders down to the calipers) must be "compatible" with the fluid wetting them. Let's take the MIL-H-5606 fluid. We all know that it is "compatible", thru testing and in use for decades with Nitrile (Buna-n) synthetic rubber O-rings.
Nitrile to its credit (it helped win a war after all) has good low temperature properties, but unfortunately has poor high temperature properties (at least when compared to newer fluorocarbon materials). Viton is such a new material. It sacrifices a wee bit of low temperature elasticity (remember the Columbia booster) for a substantial increase in high temperature margin. This is good for things that get hot (brake calipers) and not really necessary for other applications, so long as the "compatibility" thing is still there. Viton 75 durometer is compatible with both MIL-H-5606 and MIL-PRF-83282 fluids, as well as others (see Ace Seal website or other similar compatibility charts for these elastomers). Harder (greater durometer) Vitons have even less rebound at low temperatures.
Now I know that some in the Long Ez crowd and the Rocket lists say that you can use various other fluids, from DOT4/5 to Hershey's chocolate syrup, but please be prudent. It helps to know what you are doing here, because your safety is a stake. Perhaps someone's favorite fluid may in fact be a repackaging of the real stuff, but why take the chance?
The chosen fluid needs to do the job over the entire expected temperature range and be compatible with all of your O-rings. The MIL-PRF-83282 fluid was developed to replace MIL-H-5606 in all applications and all of the compatibility tests have been done five decades ago (at taxpayer expense) when the military solved this problem for their aircraft.
The O-rings merely need to be compatible with the fluid and not take a compression set at the temperature that they will experience in service. Heat soaked calipers after a down wind landing on a short strip can do a number on Nitrile. It just takes one time for them to take a compression set at excessive temperature and they are compromised (out-of-round) forevermore.
N1GV (RV-6A, Flying 883hrs, O-360-A1A, C/S, Silicon Valley)
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