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MG MGA - How to tell the difference between DOT 4 & 5?

Is there a simple way to tell the difference between DOT4 brake fluid and DOT5 (silicone) by taking a sample from the master cylinder reservoir and testing, if one is not sure what has been used in the vehicle by the the PO?
ie does one float on water and the other emulsify?
Mike Ellsmore

Mike - DOT 5 is purple in color. The other check is to get a small container of DOT 3 or DOT 4 and smell it , then compare the smell of the fluid in the master cylinder - DOT 3 & 4 have a distinctive odor. Finally, unless the fluid has been change out recently, DOT 3 & 4 will appear dirty, while DOT 5 will be clear (even after 25 years as in our TD). Cheers - Dave
David DuBois

My Automec Dot 5 is yellow-ish.
J H Cole

If you take a sample and add water the DOT 3 and DOT 4 will totally mix. The DOT 5 will not mix evenly.
Another test is to strongly stir a sample to create bubbles. The DOT 3 and DOT 4 bubbles wil soon rise to the surface. The DOT 5 bubbles are very long lasting.
This long lasting of bubbles in DOT 5 is why you have to be careful using it so as never to create bubbles. I would recommend that only new DOT 5 should be used and it should be stored in an upright container for a long period before using.

M F Anderson

If you have a scrap of painted metal DOT 4 will raise the paint on its surface. DOT 5 will not damaged the painted surface.
Keith Lowman

Actually my DOT 5 is green. I thing GE chemicals makes all of the silicone stuff but colours it according to marketers wishes.
DOT 4 will mix with water...DOT 5 won't. Also DOT 4 is a pretty good paint remover.
Spill a couple drops on your fender.You can tell right away which type it is.

G. Wayne Hardy

It's not always obvious as a paint remover, especially with some of these modern finishes. You should poor about 1/4 pint onto your bonnet/hood, then go and make a coffee. Afterwards just wipe the fluid to see if it comes straight off without leaving a mark. If it does, it's silicon based DOT5!

Neil McGurk

Nice one Neil! With your tongue stuck firmly through the last Christmas mince pie and then solidly into your cheek!
Bruce Mayo

Too right Bruce!,

Suppose you don't want to hear my tip for checking if your fuel has gone off or not then?

Neil McGurk

Just for Neil's and other's amusement I have now determined the unknown brake fluid (UBF) is glycol based.

Both my Penrite DOT4 sample and the UBF are amber colour.

I put 2 ml of DOT4 in 20 ml of water with a pipette and the DOT 4 didn't mix with the water but rather went to the bottom with an misty interface about a third of the way up the jar. I repeated with UBF sample and got the same result. When stirred both samples mixed. (Similar to the party trick with Scotch and water no mixing if the Scotch is introduced carefully).

I also tried aerating a sample of both but they did not appear to aerate.

The ultimate test was I put a drop of the UBF on the lid of my parts washer and came back half an hour later and it had started to eat into the paint.

I think my wife is going to get sick of me in retirement!
Mike Ellsmore

This thread was discussed between 05/01/2010 and 10/01/2010

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