Changing from DOT 3 to Dot 5 brake fluid?

[Greg McDonnell]

 It has been suggested I remove the DOT 3 brake fluid in my cars and replace it with DOT 5.  My cars have all had their brake systems kept up to date over the years (flushing and replacing the DOT 3, replacing components as needed, etc) and are in good working order.  On the other hand, I live in a humid climate (lower AL) and my cars sit for extended periods of time.  Should I go ahead and switch to DOT 5 and get rid of the DOT 3 fluid?  As I understand it, DOT 3 has a tendency to absorb moisture and once the moisture gets in the fluid, corrosion begins inside the components.  Is that correct?

If the consensus is I should change to DOT 5, how do I go about properly and thoroughly purging the old DOT 3 from each of my car's systems?  Is DOT 5 used to do this or is another fluid used (alcohol)?  How does one know for sure, after the purging, all the DOT 3 is removed?  If any of the DOT 3 still remains will that hurt my brake system or perhaps cause brake failure?   

I welcome all comments and look forward to any advice and suggestions!

Thank you!

Greg McDonnell
CLC#20841   

[Mike Josephic CLC #3877]

Greg:

This topic has been covered in great deetail on this forum before.  However, some
general rules:

--DOT 5 is a silicone based fluid.  Standard DOT 3 is diethylene glycol.
They are not compatable (cannot be mixed)

--Before you could switch from DOT 3 to DOT 5 you MUST change out any and
all rubber parts of your system.  That means adding new / rebuilt master cylinders,
wheel cylinders, brake hoses, power brake unit rebuilds, etc.  Flushing the system
with alcohol will not work.  On a newer car (such as those with ABS) this is a
whole additional can of worms.

--Any rubber parts that were once exposed to DOT 3 (and as a result will have
absorbed this fluid) WILL LEAK if you replace that fluid with DOT 5.

Silicone based fluid is a superior product, since it does not absorb moisture,
however the changeover is not by any means simple and requires a complete
brake system rebuild in order to insure safety.

Mike

[Greg McDonnell]

Mike,

First, thank you for your very straightforward comments.  I appreciate your being candid and laying it on the line and not sugar-coating anything.  It sounds like I would be better off just flushing my cars systems every two or three years and replacing the old fluid with new DOT 3.  I'll speak with the brake shop I use and see what they think about that.  They weren't too keen on swapping the DOT 3 for the DOT 5.  They were not sure how they would get all the old DOT 3 out of the system and expressed their concern regarding the possibility some of the 3 would remain. 

Again, thanks for your comments and advice!

Greg

[Bill Gauch #23121]

I feel that if you are up to the regular maintenance of DOT3 and don't need anything but shoes, just keep it up. The only reason I would rebuild the system is if I were doing a major overhaul of a brake system. Many times, wheel cylinders will spring leaks simultaneously. Sometimes, the master cylinder will leak too. That would be a good system to rebuild and use DOT5. Doing the changeover on a full restoration is almost a given considering you would be replacing/repairing/restoring every component on the car.

[76eldo]

I agree.  If you are doing a frame up restoration, and everything is new, use the DOT 5 silicone fluid, but for your situation, flush every two years, because you will never get all of the DOT 3 out by bleeding and flushing.

Brian

[Philippe M. Ruel]

A brake circuit is supposed to be somehow sealed, so that there is no moisture that fluid could absorb, whether you live in a humid or dry region.

The problems start with leaks : fluid gets in contact with air, every time you add fluid you open the reservoir and let air in, moisture gets inside, corrosion starts, you get more leaks, etc.

For your information, I renewed brake fluid on my car, together with new cylinders, master cylinder kit and hoses, in 1999. It was DOT3 fluid. I screwed the cap (caps were screwed on, back then in 1952) and never opened it again since then, whether to add fluid or check level (back then in 1952, master cylinders were bolted on the frame right behind the brake pedal, impossible to reach from above and uneasy from below).
The car was parked in Brittany, a very wet region of France, till 2003, starting and moving only in the summertime. Then I moved to dry southeastern France and drove it about 8,000 miles since.

Last january (2008) I noticed hoses started to show cracks, and decided to change them with new, aircraft-type, hoses. Believe me or not, fluid level was still at "max", and it still had its very-light-yellow original color. I flushed the circuit with alcohol (some dirt came out), changed the hoses and filled circuit with a can of new DOT3 fluid. That's all.

[Greg McDonnell]

Thanks everyone for your comments!  My first inclination was to do a search on the topic and read the thread(s), making a decision based on what I read.  But, after giving it some thought, I started a new thread to bring the subject to everyone's attention again for those of us who might not have been members of the CLC or visiting this forum when it was previously discussed.  I always learn so much here and in the case of DOT 3 vs DOT 5, I now know enough to say I will be keeping my systems DOT 3 but perhaps changing the fluid more frequently. 

By the way, that was a very interesting story Philippe!  Thanks for sharing it.

Greg McDonnell
CLC# 20841