ATF for clutch fluid?

Why ATF? Brake fluid is hygroscopic, absorbs water from the atmoshphere. When the temp drops the water will precipitate out, and pool at the lowest point. Galvanic action between aluminum slave/master cylinder bodies and ferrous piston causes pits to be etched into the bore, which chew the lips off of the seals, which cause the fluid to leak out.

Unlike brakes, clutch hydraulics do not see high temps, so boiling point is not an issue. ATF is specified for hydraulic duty, anti-corrosion, moderate temps, rubber seal compatible, universally available, cheap.

Silicone fluid has some issues.

Anyone try it? What results?

Not my idea BTW, proposed on another list, but sure seems to make sense.

Carter

^ You actually WANT hydroscopic oil, keeping the water "in suspension" prevents the pooling. Use brake fluid, flush it every year or two, never worry about potential seal issues. (not that I'd expect issues using ATF really)

Nathan

The seals and cups are designed for use with brake fluid. They may not be compatable with ATF. I think you are looking for a problem that doesn't really exist.

njansenv wrote: (not that I'd expect issues using ATF really) Nathan

Put oil in your brake or clutch system and let us know how it works out for you...

Last time I fixed a car where some dumbass did that I found wheel cylinder cups that had swelled to the size of mini donuts.

But, as with anything, YMMV.

Shawn

I use ATF in my carb dashpot, but brake fluid in my clutch hydraulic system.

iceracer wrote: The seals and cups are designed for use with brake fluid. They may not be compatable with ATF. I think you are looking for a problem that doesn't really exist.

+1

You talk about the expense of brake fluid... last I checked, a quart of it was only $7-8, and you need, what, less than a pint in the whole clutch system?

If $4 breaks your budget, cars are not the hobby for you, lol.

The only hydraulic clutch system I have seen that does not use brake fluid is on KTM motorcycles; they use mineral oil and state specifically not to put brake fluid in the system.

I had to tell a guy we were going to have to replace every rubber seal in his brake system because he thought red transmission fluid was compatible with pink brake fluid, that cost him a wad. ATF has petroleum lubricants in it which are hell on most any type of rubber either natural or synthetic unless the rubber is specifically manufactured to be oil resistant. I wouldn't do it.

Old jags use that goofy mineral oil in the brakes and clutch systems too. Although old jags are just goofy in general.

But, like everyone else said, you want to use the correct fluid.

Trans_Maro wrote:

njansenv wrote: (not that I'd expect issues using ATF really) Nathan

Put oil in your brake or clutch system and let us know how it works out for you... Last time I fixed a car where some dumbass did that I found wheel cylinder cups that had swelled to the size of mini donuts. But, as with anything, YMMV. Shawn

I'd be surprised if this happened with ATF (not motor oil). I still wouldn't do it. You could have problems if the seals in the system are EPDM, which could be the case on some cars.

Try it and prove me wrong!

Shawn

erohslc wrote: When the temp drops the water will precipitate out, and pool at the lowest point.

I think this is the flaw in the reasoning right here. It doesn't precipitate out at lower temperatures.

Well, I guess that's the point, it's not 'reasoning', it's observation. Been driving and racing Little British Cars since 1968, seen more clutches and brakes fail with this exact scenario than I care to count. Personally, I've always used Castrol LMA, and it's worked very well for me.

First time I saw it was with old brake fluid drained out of a system into a glass mayo jar, with the lid screwed tight. Cloudy and nasty, but no visible water. Set on the shelf above my workbench that August. Couple months later, weather starts getting cold, I notice some water pooled at the bottom of the jar.

Draw your own conclusions.

Carter

FWIW some industrial vehicles use SAE 10 or UTF for brake fluid. I have also used UTF instead of ATF for power steering fluid sucsesfully.

LBCs use a different kind of rubber, that's why they lose seals so easily. Some brake fluids attack the natural rubber seals, LMA does not. Change to better seals, the problem goes away.

I'll try the brake fluid experiment. One of the salvage cars at work will probably have some suitably nasty stuff :) But I'm not buying that it precipitates out - otherwise every car in Canada would burst its brake lines every winter as the water froze in the lines.

OK, I guess my original post was not clear enough.

We are talking about a couple of drops of water that pool at the bottom of the slave/master cylinder, just enough to bridge between the steel piston and the aluminum cylinder body (whatever the bore clearance is, maybe 0.015" ?). Galvanic action eats tiny pits in the aluminum bore. The pits have extremely sharp edges. Every time the lip of the seal wipes past a pit, it chews a tiny bit off. Eventually, your seal, isn't a seal. And of course, once a pit has started, it acts to trap and hold even more water, leading to more degradation, etc, etc.

So no, freezing temps would not burst anything.

Anyone that's ever rebuilt an aluminum slave/master cylinder has seen the pits, ever wonder why they occur, and why in only one or two places on one side?

As for the rubber composition, like I said, I've been racing and driving LBC's since 1968, I'm well aware of the genre and it's foibles. Mssrs Skinner, Lucas, and Girling have had their way with me often enough.

If you do the fluid experiment, try to find a jar small enough that there's little or no air (that would contain moisture that might condense) left above the fluid. The water I saw was like little beads at the bottom.

Carter

Though brake fluid, being hygroscopic, does NOT pool water. Silicone fluids do, but not glycol brake fluid. Brake fluid will disperse water throughout the fluid.

Fox, what goes in, can come back out.

Are you an industrial chemist? (I'm not) But I know what a supersaturated solution is. And I know what a triple point is.

If you can explain how observable liquid water can appear inside a sealed glass container of old brake fluid, please bring it. (if the brake fluid was capable of unlimited hygroscopic activity, it would simply have absorbed any water, right?)

If you have a better explanation of the pitting phenomenon, please bring it.

You know what, forget all that, lets get back on the original point.

"ATF in a brake system: Anyone try it? What results?"

So far, except for Trans_Maro, a lot of anecdotes, suppositions, and opinions.

All good stuff, but not much substance.

Carter

Carter, brake fluid is a glycol ether. You can't saturate it with water, they are infinitely miscible. Just like you can't saturate your antifreeze with water.

The only way to get water balled up unmixed in a brake fluid is to be using a silicone brake fluid. In silicone brake fluid water will pool down at the bottom. If you've got fluids balled up in a glycol ether, it's petroleum, that won't mix.

Being a geeky engineer type, I even went downstairs and performed fresh empirical testing, just for you. Brake fluid (of various types, I have several cans) in a cup, pour in water, watch it mix instantly. As is to be expected of any glycol ether.

ATF and any other petroleum oils have problems with thermally variable viscosities, cavitation, and coking at rather low temperatures. Trying to ram a semi-thick fluid through about 30 feet of fine tubing is not fun. Nor is having your caliper piston sticking because the oil is cooking off. I'm sure there are several other problems.

There is the problem with seals being incompatable with petroleum products as well. Though one could find compatable ones, or make them.

OK, at last an answer backed up by experimental procedure (although I'm betting that the fluid you used was clean).

I did some seraching on my own, and discovered that fluid manufacturers blend in corrosion inhibitors, usually boron based, up to 28% by volume. As the fluid ages it absorbs water. The corrosion inhibitor works by bonding to the water, but eventually is used up. The water then reacts with the copper in the brazing alloy which is universally used in the manufacture of the steel brake lines as part of the inner lining. The copper dissolved in the fluid serves as a catalyst to promote rapid electrolysis between ferrous and aluminum parts.

This, from an industry discussion referencing an SAE paper on the subject of corrosion damage in braking systems. (SAE paper 971007,Corrosion Prevention SP-1265, 1997)

Why it should occur at the lowest point in the system, was not addressed.

The SAE paper concluded that testing for the presence of dissolved copper in the fluid was a more reliable indicator of when the fluid should be changed, than boiling point or water percentage testing, as in fleets, etc.

The tipping point was 200 ppm of copper. Inexpensive test strips exist, the test itself takes about 2 minutes, simply by dipping the strip in the fluid.

Still does not expain what I observed ( " it moves ..."). Perhaps the liquid drops I saw was not water.

Personally, I'll stick to LMA, although stainless sleeving or plastic bodied cylinders and/or pistons certainly bear investigation.

Thanks for taking the time.

Carter

OR, you could just flush the fluid annually/bi-annually. Seriously, it seems by far the simplest solution. Re-engineering a brake system to eliminate the requirement to flush the system seems to me like re-engineering an engine in order to avoid oil changes.

I stick to my point: I use Dextron III in some dampers/struts, and only have seal issues with EPDM. Some (but not all) vehicles use EPDM seals in their brake systems which would not work at all with ATF.
I would NOT recommend the experiment in a vehicle, for all the reason others have listed.

Somebody I know (who is not me, he just looks like me and drives the same cars and has the same fingerprints and stuff) once used gear oil in the clutch hydraulics to shore up a failing master cylinder. It worked fine, in the summer time.

Backstory: A friend of mine (uh, I mean his) told me (him) about a friend of his that did that when the master in his REPU started to die, and it lasted until he got rid of the truck.

So there I was, about a year later, and 1/2 hour away from going to the dragstrip, when the clutch master in my car decided to die. No pass go, no collect $200, just flat out dead. So, the guy I knew completely flushed out the system using a vacuum bleeder and compressed air, and poured in some 75W90. Would would it hurt to try? The master was already dead, and the slave was under warranty.

Air bubbles move SLOWLY through gear oil. Spare master cap, 7/16" drill bit, tire valve, and a bike air shock pump, and I had a pressure bleeder. Bled the system out in about 5 minutes. Pushed the pedal down. It came back up! Pushed the pedal down and staretd the engine. The clutch would disengage! SUCCESS!

Well, success in the summertime anyway. This friend of mine, he lived in California. It doesn't get much cold, there. It gets cold here. When it got below about 45 degrees, the clutch action got a little "thick". When it got below 30 degrees, you could push the pedal down and count to three before it would come back up enough to let the clutch engage. Took forever to warm up, too.

But, sure enough, it let that dead master cylinder limp along for another two years before it started bypassing again. Swapped in a new master and slave and used brake fluid like you're supposed to..

Thanks for fessing up, er, I mean sharing the story about your 'friend'. In the interest of science, what kind of car did your 'friend' have?

Carter

It was an RX-7. Undoubtedly not the factory hydraulics at that point, since they are rather failure-prone. Our of four I have owned, all but one have gone through several sets, and the fourth was an automatic.

I took the cylinder apart in the name of science. The seal was jellied, and the rust in the bore was still there, just greatly polished. This seems to be the typical failure mode - the rusting bore leading to seal failure part, that is.

So I cleaned it up, stuck the seals from a good-but-wrong-style master into it, and stuck it on the shelf. Because there's always going to be another one failing...