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tubman · 03-05-2026, 02:53 PM

Actually, this thread got me thinking and I did a little research. I used to be a "DOT4 guy", but after a little digging, I'm starting to lean towards DOT3. The main advantage of DOT4 fluid is a higher boiling point (around 440° F as I recall). However, its downside is that it picks up moisture more quickly than DOT3. The boiling point of DOT3 is 407°. The way these cars are used and the very nature of the brake systems tells me that we will probably never need those extra 30°.

As things stand now, I think I'd rather have the reduced moisture pickup and retention than the extra protection offered for "heavier vehicles and trailer pulling".

Speaking of moisture in brake fluid, I purchased a tester about 15 years ago that will determine the moisture content of your DOT3 or DOT4 brake fluid. I use this every year to check my vehicles. I have found that for collector cars stored under favorable conditions, DOT4 brake fluid doesn't reach a minimum moisture level until after 9 years. If DOT3 extends that even a year or two, I'll start using it, because I don't use my old cars to pull trailers and I hate changing brake fluid.

I put DOT5 in a '68 Corvette I owned years ago. After I got it in and bled, it was great, but not near worth the effort.

03-05-2026, 02:53 PM	#16
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 12,148	Re: 1950 Ford Custom Brake Master Cylinder Actually, this thread got me thinking and I did a little research. I used to be a "DOT4 guy", but after a little digging, I'm starting to lean towards DOT3. The main advantage of DOT4 fluid is a higher boiling point (around 440° F as I recall). However, its downside is that it picks up moisture more quickly than DOT3. The boiling point of DOT3 is 407°. The way these cars are used and the very nature of the brake systems tells me that we will probably never need those extra 30°. As things stand now, I think I'd rather have the reduced moisture pickup and retention than the extra protection offered for "heavier vehicles and trailer pulling". Speaking of moisture in brake fluid, I purchased a tester about 15 years ago that will determine the moisture content of your DOT3 or DOT4 brake fluid. I use this every year to check my vehicles. I have found that for collector cars stored under favorable conditions, DOT4 brake fluid doesn't reach a minimum moisture level until after 9 years. If DOT3 extends that even a year or two, I'll start using it, because I don't use my old cars to pull trailers and I hate changing brake fluid. I put DOT5 in a '68 Corvette I owned years ago. After I got it in and bled, it was great, but not near worth the effort.