Brakes: Switch from DOT 3 to DOT 5 brake fluid

[Mark L]

I'm about to break into the brake system on my 62 Lark Daytona to replace the master cylinder with a dual piston unit and replace the rear flex hose. I was thinking about switching from DOT 3 fluid to DOT 5 fluid since I'll be having to completely refill the MC, replace the original single brake tube coming off the MC, installing a new, second tube off the MC, and replacing the rear flex hose (I'll be introducing a lot of air in the lines that will need to be bled out).

I've seen some posts on the Forum made by others who've switched from DOT 3 to DOT 5 by simply making sure all the old fluid was flushed out of every component and refilled and bled with DOT 5.

Question: If I do not replace the wheel cylinders that have been used with DOT 3, will I have any problems down the road with the seals failing in the wheel cylinders after the switch to DOT 5?

[rbisacca]

Many years ago I switched from DOT 3 to DOT 5 in my '62 Champ by simply flushing and refilling and have had absolutely no problems.

[Skip Lackie]

I'm sure you will get several opinions, but I think to be safe I would replace or at least disassemble/flush/reassemble the wheel cylinders. I have heard a number of stories of rubber parts becoming gunked up by the mixture of DOT 3 and 5. Opening up a brake system should be sufficiently rare that any needed maintenance should be done at that time. Rubber parts should be clean when the DOT 5 is introduced.

[stude1964]

I've always done a thorough bleed and have had no problems. I switched 5 vehicles to DOT 5 for years and no problems. As for the stoplight switch I put a couple drops of the old style fluid before installing. I haven't done a mechanical switch yet and really not planning on it. I always hit the brakes before starting and check for the ammeter to move if I can't see the brake lights for myself.
Rob in PA.

[Studebaker1965]

Rob, do you put a few drops in the switch before installing it in the dot 5 system? I’m about to switch my car over to dot 5. Doesn’t that cause the switch to gunk up?

[unclemiltie]

I always change all rubber parts including hoses and wheel cylinder/master cylinder rubbers when making the change. I have changed close to 20 cars to dot5 in this manner and never had an issue. When I have the lines all undone to replaces hoses etc I blow the old fluid out well Some say you flush with alcohol but I have never done that.

[stude1964]

Yes-a few drops of the DOT 3 fluid into the switch.
Rob in PA.

[(S)]

You can put a mechanical stop light switch on there without too much trouble. For one, it's cheap insurance. For two- you can ADJUST it like a modern car so the lights come on as SOON as you start to push the pedal. You can give the driver behind you a full second more warning.....

In any system, bleed the brakes every few years or sooner if you have Dot 3......

[StarDiero75]

Whats the benefit of DOT 5 over DOT 3? Im gonna be redoing the ENTIRE brake system (from new MC to lines (metal and rubber) and wheel cylinders), so should i do 5 instead of 3?

[TWChamp]

Whats the benefit of DOT 5 over DOT 3? Im gonna be redoing the ENTIRE brake system (from new MC to lines (metal and rubber) and wheel cylinders), so should i do 5 instead of 3?

DOT 5 doesn't attract moisture like DOT 3 does.
I change to DOT 5 in all my antique cars and trucks when I do a brake job on them.

[jnormanh]

Whats the benefit of DOT 5 over DOT 3? Im gonna be redoing the ENTIRE brake system (from new MC to lines (metal and rubber) and wheel cylinders), so should i do 5 instead of 3?

Comparison:

Upside: DOT5 does not absorb water from the atmosphere, boils at a higher temperature, and probably lubricates rubber parts better.

Downside: DOT5 does not cause the minor swelling of rubber parts that DOT3 does, and therefore is slightly more likely to leak.

In the case of a minor leak, DOT3 may evaporate and not cause a problem whereas any amount of DOT5 will not evaporate and is slicker than snot.

A DOT3 system which is ignored for years may develop significant rust. Dot 5 will not.


I have used DOT 5 in a newly rebuilt system but converted back to DOT 3 because of multiple tiny leaks. DOT 3 never leaked in the same car.


Personal opinion: I would use DOT 5 in a car which may sit for months or years, but I would use DOT 3 in a car which is driven once a month or more often.

If you change from one to the other, you should replace all seals and do a thorough system flush.

[(S)]

Star Diero 75, You will want to use it, if you are in Western WA.

[StarDiero75]

So i do plan on driving it semi regularly, so more than once or twice a month. Then i shouldn't switch then? But if i drive it rarely then DOT 5 is the way to go?

[jnormanh]

So i do plan on driving it semi regularly, so more than once or twice a month. Then i shouldn't switch then? But if i drive it rarely then DOT 5 is the way to go?

That's what I would do. Others may have different opinions.

[bensherb]

I've used DOT 5 since the 70's and the ONLY disadvantage I've found is, it will leak (actually more correctly seep) at fitting/hose connections IF they are not properly torqued to spec. With fittings/hoses properly torqued DOT 5 will not leak.

A big advantage to DOT 5 is it will not damage paint, in fact you can shine up your finish with it. I believe "sweetolBob" can attest to that. Not a big thing for a car with master and everything else below the floor, but with the master on the firewall, or a motorcycle with a master on the handlebars it's a godsend. In fact Harley Davidsons have come factory with DOT 5 fluid for decades.