Is there a definitive way to tell if you have silicone brake fluid in your car?

Is your pedal firm and everything works okay? Silicone is compressible so it can seem spongy. If it's not you're probably fine.

Just flush it out and refill with DOT4. I've done it before without problems.

 

take some of the fluid out of the master cylinder place it in a container then add some DOT-4 to the same container. Shake the container to mix them up and then let it sit and see if the 2 separate. DOT-4 and DOT-5 will separate as they are not compatible.

If you have DOT-5 in your system and you want to switch to DOT-4 you will need to rebuild the entire system replacing all of the rubber components.

 

One of the stories about DOT5 is that you have to rebuild everything if you change to "real" brake fluid.

I'm suggesting that I've simply flushed it out with DOT4 and it was okay afterwards. The DOT specs require compatibility. If there's trouble later you can always rebuild it then. I've fixed quite a few 40+ year old Jaguar brake systems.

 

find a painted piece of metal you don't mind messing up.
put a sample of the brake fluid on the painted surface and let it sit for about 20 minutes.
If the paint disolves or is otherwise really messed up, you don't have DOT5.

DOT5, being silicone will not attack the paint, whereas DOT4 (and lower) will attack paint.

JD

 

DOT-5.1 will eat paint also as it is not Silicone based.

 

A number of years ago, I did a restoration on a '69 E-type roadster at a shop in Pennsylvania that I've used for many years. We completely rebuilt the brake system. When I picked the car up, the mechanic told me that he had put DOT 5 in, because the silicone fluid would not mess up the frame rails and paint work if any dripped from the fluid reservoirs. He put DOT 5 in the clutch reservoir as well. Apparently, this was fairly common with Jaguar restorers because of the location of the reservoirs up next to the engine, sitting over all those painted components.

Agree with the "tests" you were given. I'd try both, just to be sure -- the separation test and putting some on a painted surface. And DOT 5.1 is compatible with DOT 3 and 4. DOT 5 (silicone) is not.

IMHO, if you have silicone in the system, live with it. It's not bad for ordinary street use. But you should bleed the brakes once a year to be sure you don't have any water building up in the system. DOT 3/4/5.1 you can bleed every two years for ordinary street driving, unless it is extremely humid where you live, then do it every year. YMMV, but that's my take on it.

 

By the way, nice car!!! I currently have a '70 E coupe, midnight blue/black. I'll post a picture later, if I can dig one up.

 

Try putting some water in it. Silicone will not mix with water. Regular fluid will

 

Pretty car. Every question involving a specific car should include a picture. cheers.

 

Me and the '70 XKE crossing the finish line at "The Brickyard" (notice the strip of bricks). At a national Jaguar club event a couple of years ago, we got to do parade laps on the Indianapolis Motor Speedway, which was quite a thrill.
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Aaron- Forget the tests and do not worry about it. Just replace all the brake fluid like we do every year for Ferraris. Completely drain and flush the system and then bleed the calipers, clutch (if applicable, do not know squat about E-Types) and master cylinder (if possible) a couple of times and all the silicone brake fluid will be expelled. Not worth worrying about it when you can fix it and be up to date on that particular service.

Like a pair of pants with something soaked into them that irritates your skin. Change pants and wash or throw away the bad ones. No sense analyzing what irritated your skin.

Taz
Terry Phillips

 

For what it's worth DOT 5 (silicone) brake fluid is Purple in color by law. You should be able to tell the difference by looking at it.