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Brake fluid change DOT4 to Royco782

Tam

Member
Dear All,
Having recently acquired a 7years old RV8, we found out the brake fluid used was DOT4 automotive fluid.
Since this fluid had never been changed in this 7years period, we decided to drain it and replace by royco 782 fluid.
Since then, brake pedals are much more mushy than they were before although still efficient.
Did we do anything wrong?
many thanks in advance for your advises

Tam
 
The obvious possibility is incomplete purge of air bubbles - aka bleeding.
Not sure of compatibility issues between fluids.
 
I question if the seals that work with DOT4 and Royco782 are compatible. I would want to have used all new O-Rings / seals with the fluid change.

As stated above, there is a possibility of air in the lines.
 
From the Royco 782 Data Sheet:

"ROYCO 782 is a red - colored synthetic hydraulic fluid developed to meet the severe duty demands of today's
military and industrial equipment. Modern additive technology is employed for oxidative stability, corrosion inhibition, and antiwear protection. ROYCO 782 is compatible with petroleum-based fluids and all components
used in mineral oil type hydraulic systems."
 

Attachments

  • Royco 782 Data Sheet.pdf
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Air in the brake system

The glycol and mineral fluids are NOT compatible. Sorry to say, you will need to tweak your bleeding system to make sure no air is introduced, and any air in the system is expelled.

Dot 4 will last a very long time unless you “cook it” as in drag the brakes etc. Certainly, you will know if you have done that by the fluid under the tire (O-rings cooked). Auto type brake fluids are not the best choice - especially if you are out in the boonies with no brake on one side..most airports would have someone who has the red stuff for you to use.

I think it’s much better to use mineral fluids, due mainly to the availability of the same fluid in case of a leak in the middle of nowhere - or anywhere else.

Mobil 1 ATF (available at any Walmart) is also NOT compatible with glycol-based auto brake fluid - BUT - we all know how prolific Walmart stores are - so if you have a leak on a trip with Royce/ATF/Mineral in your system, you can refill with fluid from Walmart (the fluids are compatible). Also, Lowes/Home Depot carries the caliper O-rings if you have REALLY over-cooked the system (plumbing area). Change those o-rings to better quality when you get home - or not - those will last to your next annual condition inspection - you can change the O rings then - unless you overcook those too!!:eek:

This particular system contains no Rocket Science! Fear not!:D
 
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Ummm,no. DOT 4 is definitely not ATF! DOT 4 is totally compatible with DOT 3,it simply has a higher boiling point. It is not compatible with oil based fluids at all.
 
Ummm,no. DOT 4 is definitely not ATF! DOT 4 is totally compatible with DOT 3,it simply has a higher boiling point. It is not compatible with oil based fluids at all.

+1

DOT 3/4 is polyglycol based (4 has borates added to increase boiling point)(DOT 5 is silicone based). Most aviation hydraulic fluids are mineral oil based, including both Royco brake fluids. Fluilds are NOT compatible, though not sure slight mixing will cause issues. Bigger concern is seal incompatibility. I would expect that the Buna-N used in your system may not tolerate glycol based fluids, though not sure. DOT 3/4 can be nasty to other stuff; it is a known paint stripper. I would do the research and if incompatible, I would replace all of the rubber in the system.

Do agree with others that the issue here is likely air that has not been removed.

Glycol based fluids capture and retain moisture and require closed systems to avoid corrosion problems. This likely why it is not used in aviation. It also necessitates routine fluid changes to evacuate that moisture. These are not necessary with mineral based systems that do not support moisture in suspension.
Larry
 
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Many thanks for all your advises and expertise
Not sure what’s right or wrong but when we picked-up the plane (with DOT4), the brakes were really hard, pedals almost not moving at all.
Now (with new fluid), they are more mushy but seems more «*normal*» feeling to me. how is it on your planes?
Thanks in advance
Tam
 
Many thanks for all your advises and expertise
Not sure what’s right or wrong but when we picked-up the plane (with DOT4), the brakes were really hard, pedals almost not moving at all.
Now (with new fluid), they are more mushy but seems more «*normal*» feeling to me. how is it on your planes?
Thanks in advance
Tam

The system should be hard as a rock, like what you had before. Fluid type makes little difference. Bleeding these systems takes a bit of work and finesse. I have to shake the masters like crazy and pump while bleeding to get all of the air out.

IMHO, you didn't bleed them well enough and that's why they are soft - air in the system.

Larry
 
A bit more on seals, which makes for a "that depends" answer to the OP's original query. In the automotive world where one finds DOT 3, 4 and 5.1 (not 5), EPDM rubber is the typical go-to seal material. It has very poor resistance to hydrocarbon liquids; it goes soft, swells and eventually melts.

If the brake seals in the -8 were converted using that, then you are going to get poor sealing with internal and external leaks eventually. Could explain the soft pedals, though if that happened right away then I'm with the other posters- most likely didn't get all the air out.

On the other hand, if the seal conversion was done with Viton (a fluoropolymer that's immune to most common liquids found in cars and airplanes and also the most expensive common seal material) then you are good to go. Were it me, unless the seal material can absolutely be verified I'd change all of them out of general principle.

I converted my RV-7 brakes to DOT4 right away (yes, I used a sealed reservoir). Wouldn't do it again; I'd use Royco and be done with it. Unfortunately I can't remember what seal material I used or I'd just flush and be done with it. Too lazy to change all the seals now, since the brakes work.
 
Not an expert

If it were me, I'd flush the DOT, fill with Royco, flush, refill, maybe flush a last time then change the seals and do a proper bleed...
 
brake fluid compatibility matrix

If it were me, I'd flush the DOT, fill with Royco, flush, refill, maybe flush a last time then change the seals and do a proper bleed...

I found this a while back. I'd probably just upgrade to DOT 5.1 (which is what I run, after changing all the seals.)

brake-fluid-compatibility-chart.png

From here: https://epicbleedsolutions.com/blogs/faq/whats-the-difference-between-dot-4-and-dot-5-1-brake-fluid
 
Different O-ring compounds used

Automotive brake fluids require the use of EDPM O-rings. EDPM is rated for 300F temperatures. The original O-rings are Nitrile [aka Buna N] which are rated for 250F. For the fluid you want to use, switch the O-rings back to Nitrile. That said, I would suggest using Viton O-rings in the calipers, as it is compatible with the Royco fluid and is rated for 437F. Viton will also work with automotive brake fluids, but is not as resistant, as EDPM.
Viton is overkill for the parking brake valve and master cylinders. It is also slightly softer than Nitrile, so will wear more rapidly. Does your 7 have Matco calipers? Or the older Cleveland units? The O-rings are different sizes for each manufacturer.
FYI, 7 years is way to long for DOT 4. It should be flushed every 2 years.

Charlie K

PS Use the web link below for determining O-ring material suitability with various fluids

https://www.allorings.com/o-ring-compatibility
 
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Automotive brake fluids require the use of EDPM O-rings. EDPM is rated for 300F temperatures. The original O-rings are Nitrile [aka Buna N] which are rated for 250F. For the fluid you want to use, switch the O-rings back to Nitrile. That said, I would suggest using Viton O-rings in the calipers, as it is compatible with the Royco fluid and is rated for 437F. Viton will also work with automotive brake fluids, but is not as resistant, as EDPM.
Viton is overkill for the parking brake valve and master cylinders. It is also slightly softer than Nitrile, so will wear more rapidly. Does your 7 have Matco calipers? Or the older Cleveland units? The O-rings are different sizes for each manufacturer.
FYI, 7 years is way to long for DOT 4. It should be flushed every 2 years.

Charlie K

PS Use the web link below for determining O-ring material suitability with various fluids

https://www.allorings.com/o-ring-compatibility

Charlie, I bought my DOT4 conversion o-rings from you way back in the day. What material did you supply (ref my previous post above)? For what it's worth, I rarely let any of my arcane collection of old cars and motorcycles go more than a year without purging, and even then sometimes old fluid comes out nasty and dark. Yet, I recently needed to flush the DOT 4 in my RV due to allowing a caliper piston to leak. Amazingly, it was as clear as when I first filled the system 8 years and over 1000 flight hours (mostly short hops, lost track of how many landings) ago. Go figure.
 
When you go conventional Mil 5606 or Royco 756 hydraulic fluids (they can be mixed) then you are "compatible" with most all aviation maintenance shops and suppliers, especially when you land hundreds or thousands of miles from home and have brake trouble. Any A&P or friendly hangar rat can hook you up with their pressure pot to bleed your system. You are also compatible with the O-rings and/or seals that come from the brake system manufacturer right out of the box.
 
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Charlie, I bought my DOT4 conversion o-rings from you way back in the day. What material did you supply (ref my previous post above)? For what it's worth, I rarely let any of my arcane collection of old cars and motorcycles go more than a year without purging, and even then sometimes old fluid comes out nasty and dark. Yet, I recently needed to flush the DOT 4 in my RV due to allowing a caliper piston to leak. Amazingly, it was as clear as when I first filled the system 8 years and over 1000 flight hours (mostly short hops, lost track of how many landings) ago. Go figure.

Lars,
The O-ring kits are 70 durometer EDPM. FYI, I've found a reasonably priced tester for automotive brake fluids. It has 5 LED lights. Green = 0% moisture. One Yellow = 1% Moisture, Two Yellow = 2% Moisture One Red = 3% Moisture [recommended replacement at 3%] Two Red = 4+% Moisture.

https://www.amazon.com/ITEQ-Liquid-Tester-Indicators-Calibrated/dp/B076SC377J/ref=sr_1_2?crid=3OO0VTG0CLSXZ&dchild=1&keywords=brake+fluid+tester+dot+3+dot+4&qid=1608063983&sprefix=Brake+fluid+tester%2Caps%2C228&sr=8-2

I bought mine just before Halloween of 2016. Price is now less than 60% of what I paid back then. FYI, in 4 years, it's still on the original battery.

Charlie
 
many thanks for all your inputs
we have the cleveland brakes and the original Nitrile O-rings
when we flushed the old DOT4, was nice and clear.
we bleeded the system once more, did a taxy test and looks good...
Tam
 
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