Roger Walling

What is the Difference between DOT 4 and DOT 5.1 Brake Fluid?
DOT 4 and DOT 5.1 are both glycol-based brake fluids and are used widely in the automotive and cycle industry. They are controlled by standards set out by the Department of Transportation (DOT) - hence the name.

The main difference between these two brake fluids is in their boiling points. Part of the standards that need to be met by the manufacturers of DOT fluids are the minimum dry and wet boiling points. These are the minimum temperatures that the brake fluid must perform at before the brake fluid starts to boil, which can lead to complete brake failure.

Let's take a look at the minimum boiling temperatures of DOT brake fluid as specified by the Department of Transportation.

Various boiling temperatures for DOT brake fluid
Remember, these are only the minimum standards. Brake fluid manufacturers can and often do improve on these figures and it is possible to find DOT 4 brake fluid with a higher boiling point than some DOT 5.1 fluids on the market.

Since DOT 4 and 5.1 are both glycol-based brake fluids they are compatible with each other, which means they can be readily mixed without harming your brake system. It is important never to mistake DOT 5.1 (glycol-based) with DOT 5 which is silicone-based and should never be mixed with any other DOT fluid.

So just which brake fluids can you mix without causing harm to your brake system? Let's take a look at the chart below.

Compatibility chart for DOT and Mineral Oil brake fluids
Here you can see that silicone based DOT 5 is the odd one out and is not compatible with any other DOT brake fluid. By mixing DOT 3, 4 and 5.1 brake fluids, assuming it is fresh fluid, the worst thing that can happen is a drop in the boiling point of the whole fluid.

Some brake manufactuers, such as Hayes and Formula, pre-fill their brakes with DOT 4 brake fluid from the factory. Others including Avid and Hope, choose to use DOT 5.1 in their brakes. Many riders with DOT 4 in their brakes will opt to bleed with DOT 5.1 to benefit from the higher boiling point and improved heat resistance.

To see charts, go to below site

http://www.epicbleedsolutions.com/re...1-brake-fluid/

66jack

Some good info there...

I can see someone actually mixing/messing up on the 5.1-5.0 conundrum...

John BX NY

Not sure if it relates to motorcycles but SAE paper 810804 disagrees. It says "Brake performance of systems in which silicone brake fluid and conventional brake fluids become mixed will operate normally with no fluid or elastomeric related problems."

So when I changed my 67 to silicone fluid many years ago I flushed the system through with the silicone fluid and have had no problems for over 20 years.

rfn026

Silicone fluid has become controversial in the past few years. It seems the formula was changed and it's no longer compatible with the current seals. I had a problem with the m/c on my '58.

White Post Restoration and Lonestar Caliper both recommend avoiding silicone fluid. I know a lot of people here love it though. I loved it for several decades.

If you flush your brake system with new fluid ever two years you should never have a problem. When it comes to temperatures there is no need for the high-temp fluids on the street. Here's a recent article I did on brake temperatures.

The brake fluid actually never gets much above 250-300 degrees. Even if the caliper is at 1100 degrees the actually fluid never gets much above 300-degrees. You can measure it by inserting a thermocouple into the caliper.

In my '58 I'm using the old fluid that Ford developed for the GT40 at LeMans back in the day. It's cheap and I can buy it on Amazon.com

Richard Newton

tubman

I have DOT 5 in my '68 convertible. It works good now and is worry-free, but it was a major PITA to get bled. I am running DOT 4 in my '67 coupe and am very happy with it. There are two things that make me prefer the Glycol-based fluids of over the silicon-based ones these days. The first is "Speed-Bleeders", which make bleeding super easy, and the second is a reasonably priced electronic tester that will tell you the moisture content of the fluid in your vehicle. I test my vehicles every couple of years; when I see the moisture content starting to climb, I change it, which is an easy job with the speed bleeders.

I have found that it takes about 10-15 years for Glycol-based fluid to start showing any significant amount of moisture. This assumes that the cars are used under ideal conditions : always garaged, only driven in bad weather when it can't be avoided, and low mileage (<5000 per year).

EDIT : Richard Newton, I used to flush my systems much more frequently than I do now. Also, it's satisfying to know exactly what's going on in the system.

juanvaldez

I use DOT 5.1 in my bicycles. Did y'all know that brake fluid actually has suspend air in it?

watson

I have been using the silicone based fluid in both of my C2's for years with no issues. I started using it because it was part of the Muskegon Brake caliper kit that I put on both cars. I have always liked the safety feature in terms of paint damage if I spill some on the fender or firewall. What are the problems that you folks are concerned about?

Thanks,

Doc

rfn026

The newer formula seems to create swollen seals. That's what the rebuilders are finding.

Richard Newton

watson

Thanks, is the new formula the same for all brands or is the issue with some and not others?

Doc

rfn026

They tell me that it's a problem with all the different brands.

Richard Newton

wmf62

what, specifically, is it?

FWIW, Castrol makes a LMA (low moisture absorbing) fluid; don't know how true it is, but it sounds good...
Bill

427/42566 coup

I switched to Silicone many years ago. I replaced calipers and rebuilt master cylinder. No problems

Factoid

Hey, let’s have some fun and pull up old threads about what motor oil to use!

Frankie the Fink

And, regrettably, member Richard Newton has passed on....he wrote some fabulous articles for the hobby..

MikeM

Or anti-freeze threads. Specifically Dex-Cool threads.