Scotty,
I have a 2003 Ford F-150 with the bulletproof 4.2 v6, and the Mazda sourced 5 speed with 280k miles on it. I had a rear main seal leak, and I took your suggestion to try some AT-205. It worked.
I still have a drip, but it’s not engine oil. It’s brake fluid, which this truck uses as hydraulic fluid for the clutch. There is a slow leak that I have been adding brake fluid every few tanks of gas for the last 100k miles or so.
I’m very tempted to add an appropriate amount of AT-205 to the clutch master cylinder reservoir. The bottle does indicate that it is good for hydraulic systems.
I hesitate because there is a bold warning to not use this product on brake systems.
so, here is the question. Is that warning because this is not compatible with Dot3/Dot4 brake fluid? Or is this a lawyer talking?
We have been asked a similar question before, only about brake systems themselves. If the system takes hydraulic pressure like a brake system, no, don't use it. The AT-205 is basically brake fluid, in your case, it likely won't help. Replace the clutch master cylinder. At-205 is a band-aid.
https://carkiller.com/scottykilmer/qa/can-you-use-at205-in-your-brake-system/#post-138206
How brake fluid seals or rejuvenates gaskets is beyond me.
It very well could be incompatible chemically with brake fluid - it could also be that making the brake caliper piston seals swell could cause the calipers to get stuck - in either full braking or no braking position. It could very well make your clutch slave cylinder seals swell enough to freeze up your clutch.
It's not that hard/expensive to replace the slave cylinder and hoses, unlike engine and trans seals. You know that is what you need to do. You've gotten a lot of good miles out of your F150, give it the attention it needs.
If it says not to use it on brake systems, don't use it on brake systems.
Same with hydraulic clutch systems.
How brake fluid seals or rejuvenates gaskets is beyond me.
It makes sense if you think about it in a slightly different situation.
I believe DOT 3 and DOT 4 are alcohol based, which is why they absorb water from the air.
In order to run on E85 safely, cars must have alcohol-proof rubber and metal fuel components to carry pressurized fuel. If not, the alcohol will attack rubber and degrade it (softening it). That can cause ruptures, collapsed lines, damaged injectors, etc. Carburetors before they introduced ethanol-laced gasoline in the 1970s suffer the same thing if regular pump gas is put in them. The alcohol softens the rubber and causes problems. My '79 Catalina doesn't have that problem so much, because it was originally designed for unleaded gas.
AT-205 in the oil pan causes the rubber it comes in contact with to swell the same way the alcohol in E85 does (it's slightly attacking the rubber) but the pressures in certain systems where it's being used aren't enough to push a softened seal out.
Fascinating. Thanks for the detailed explanation.
I gather the same logic works for the “Scotty” use of AT-205 on CV Axle boots and bushings?
I just read up. Apparently ethanol doesn't plasticize rubber like DEG (Diethylene Glycol), it embrittles it, which is why it destroys incompatible fuel systems. DEG as @MMJ said is the plasticizer component of brake fluid, which is an alcohol, though we don't think of it as such. https://thechemco.com/chemical/diethylene-glycol/
So my theory was close, but not quite accurate.