Re: D.O.T. 3-4 vs. D.O.T. 5 for a '56 Patrician

Posted by bkazmer On 2015/9/16 12:21:07

I do claim to be some kind of chemical eng'eer (opinions differ on what kind), so I'll give it a try

DOT3 - ether based. boiling spec 401 F/ 284 F dry/wet

DOT4 - ether based. boiling spec 446/311

DOT5 - silicone based. boiling spec 500/356

DOT 5.1 - ether based. boiling spec 500/356

So what?
DOT 3,4, and 5.1 are compatible with each other.
DOT5 is not compatible with the others. you know that.

higher boiling point matters if you are braking hard a lot.
the boiling point goes down as the moisture is absorbed as shown with the specs. probably not an issue with most of our Packards.

DOT 3,4,5.1 absorb water up to a point - this is not a bad thing in the short term, it prevents free water. They all damage paint. Longer term it means they need to be flushed.

DOT 5 absorbs very little water. It does absorb air more easily and clears air bubbles less readily. Thus the tendency to more compressibility and softer pedal.

Free water can quickly cause local corrosion