Brake Fluid for PA12 Brakes with Scott Brake Cylinders

My PA12 brakes are getting soft. I suspect more brake fluid is needed. When I went to buy the brake fluid I see two different mil specs... 5606G and 5606H. The "G" version apparently is to be used in systems with natural rubber parts whereas the "H" version is for use in systems with synthetic rubber (whatever that is???). Don't know whether my Scott brake cylinders and the bladders in the expander brakes are natural or synthetic rubber. Can someone give me some guidance regarding which of these two mil spec version I should be using?

looks like 56o6G has been super seeded by 5606H
http://www.everyspec.com/MIL-SPECS/MIL-SPECS-MIL-H/MIL-H-5606H_NOTICE-1_27043/

5606 is petroleum based. I think that the stuff that you are referring to for natural rubber is vegetable oil based. Something that went away with the wright flyer. Is your brake oil RED?

As far as I know both 5606G and 5606H are red. My AP/IA said I could use either in my PA12. Actually found that Univair stocks 5606A (which was replaced by 5606G so that is what I ordered.

Experimental PA-12 with Cleveland brakes and old Scott cylinders:

Will regular automotive brake fluid work okay?... or, I know some brakes use automatic transmission fluid (red). Might be hard for me to get aviation-type fluid shipped in here by air and automotive stuff is readily available.

Forgive my ignorance - first time on wheels!

Automotive fluid will ruin the rubber components in an aircraft brake system. It causes them to swell dramatically.

The early j3 and j4s used brake fluid because they had automotive type cups in the master cylinders. The expander tubes can handle either 5606 or brake fluid but you can not mix the 2 together because they will jell into a snotty goo. If you want to switch from one to the other you have to flush the system with isopropal alcohol. You will have to remove the expander tubes and fill them with isopropal and knead the tube and flush and knead a few times to get it claened out. Maybe Bob Turner will chime in, I think he has been mixing dot 5 silicone brake fluid with both and not had any problems

Glenn

Interesting thing - Hegar brakes say the same thing about automotive brake fluid (that it swells the rubber), so they insist on ATF (!!). What say y'all?

Is 5606 ATF. And if it is, what type, Dextron or type F

Glenn

I do not mix fluids in brakes. I just mixed them on the workbench to see what would happen. I saw no gooey messes.

I currently use Dot 5. It is benign, but extremely expensive. It also has a Mil Std number if that impresses you. The TCDS does not specify what brake fluid to use, but Piper used automotive.

Before I went to silicon fluid I used Dot 3/4 in the J-3. My expander tubes reacted with displeasure when a mechanic convinced me to use 5606 in the 1960s.

Dot 5.1 is like Dot 3, not like Dot 5. They are just trying to confuse you.

On the master cylinders - I use Grove, but if you still use diaphragms, make them from Buna N, and they will not care what fluid you use. Leave the bottom screw out, and use 8:32x1/2 socket head screws elsewhere. Carry a spare diaphragm and a ball end hex driver, and you can change a diaphragm in ten minutes flat. If you are stuck in Ogallala, you can use a truck inner tube for your diaphragm. Ask me how I know that. Train watching is great in Ogallala, and I was celebrating escaping from Stapleton's TCA without a transponder.

I use 5606 in the Decathlon. I do not like it, but since I fly twice a week it does not get all congealed and messy. I was going to convert, but Dot 5 went from five bucks a quart to thirty!

One more hint - all it takes to make a diaphragm is a pair of shears and one of those inexpensive deals that makes holes in your belt. And another ten minutes. Trivial, and legal.

It would be interesting to know if the red "brake fluid" is just re-packaged automatic transmission fluid, with the price jacked-up of course!

Nope. Vast difference. Pour a sample of each into a jar lid and let them sit for a week. 5606 will be a sticky mess.

Even different ATFs are not compatible - Ford takes type FA and Chevy takes something else.

bob turner said:

Nope. Vast difference. Pour a sample of each into a jar lid and let them sit for a week. 5606 will be a sticky mess.

Even different ATFs are not compatible - Ford takes type FA and Chevy takes something else.

Click to expand...

Apparently, because of exposure to air.

I'm confused. So, 5606 IS ATF? ...and that's why it becomes a sticky mess? Or 5606 IS aviation brake fluid (hydraulic oil?) and it doesn't matter that it becomes a "stick mess" in the brakes and lines?

5606 is definitely not ATF. It is aviation hydraulic fluid.

Years ago I used ATF in a pinch to get my plane flying, but the had ATF expand and lock down the brakes on the runway as the brakes warmed up. Don't know why it didn't just bleed back through the master cylinders, but it wouldn't. I had to open the bleeders on the brakes to get them to unlock so I could get the plane off the runway. As I taxied back to the hangar, the brakes locked down again so I had to bleed them again to push the plane into the hangar. I flushed the system and replaced the ATF with 5606 hydraulic fluid. Never had another problem with it in the last 1000 hrs. 5606 doesn't have the thermal expansion rate of ATF.

You can get away with ATF in a pinch with some brake systems, but it is definitely not the same as 5606.

-CubBuilder

Okay.

I've been using ATF in the Hegar brakes on another airplane for the last eight years without any problem. In fact ATF is the recommended fluid for those brakes.

The Clevelands may have different requirements, so I'll use 5606 if I can get my hands on some.

Thanks all for the info.

From the Aircraft Spruce website regarding 5606H:

"This product can be used in both aviation and non-aviation systems with synthetic rubber components, but should not be used in systems that incorporate natural rubber."

I think this goes to the original post in this thread.

The old Scott masters (probably?) have natural rubber diaphragms (?). The Cleveland brakes have...synthetic or natural rubber?

Back to what brake fluid to use? :roll:

NunavutPA-12 said:

From the Aircraft Spruce website regarding 5606H:

"This product can be used in both aviation and non-aviation systems with synthetic rubber components, but should not be used in systems that incorporate natural rubber."

I think this goes to the original post in this thread.

The old Scott masters (probably?) have natural rubber diaphragms (?). The Cleveland brakes have...synthetic or natural rubber?

Back to what brake fluid to use? :roll:

Click to expand...

Hm.....the 1941 J3 that I fly has had 5606 in the system for 72 years

Glenn