Which is best- DOT 3 or DOT 5 silicone brake fluid?

[Henrius]

Tried to drive my 1965 Fury for the first time the other day, and Yikes! Found I had no brakes! Don't know where the brake fluid goes when a car just sets up for quite a time.

I have had this car on silicone brake fluid, but a Tech school auto shop instructor had doubts this was the best bet. I have always been told silicone is more inert and less likely to absorb moisture from the brake cylinders. Should I use DOT 5 again, or go back to the car's original brake fluid?

 

[live4theking]

I was told not to mix and match because if you do they will harden.

 

[Henrius]

I was told not to mix and match because if you do they will harden.

Yeah, I know it is bad to match. When I converted to silicone, I flushed the system with rubbing alcohol before putting the DOT 5 through the lines. I could flush again and go back to DOT 3, or continue with silicone. Don't know which is best for our older cars.

 

[Zymurgy]

I would stay with the silicone you have some other issue going on. It would be empty if it was DOT 3 too. I have many people I trust, say to use the DOT 5. I have had it in a '73 non Mopar for 35 years, replaced the master and rubber lines once in that time period. It sits from November to April and is driven around 1,000 miles a year.

 

[saforwardlook]

When I do my restorations, I replace everything in the system except the brake lines themselves unless there is a rust issue with them or any uncertainty with them at all. I have been using silicone fluid in each of the cars for as long as 30 years with no issues at all, and this is the case with even some of my Forwardlook cars with total contact/centerplane brakes that are a real pain to keep working well with DOT 3 regular fluid. There was an issue with some of the fluid pressure activated brake switches in the master cylinders, however, that would cause the switches to not funcion, but never a leak in the brake system. I changed over to a mechanical brake switch system from later year cars and that took care of the problem.

I would not, however, just flush out the system and then install the DOT 5 fluid, as I have heard other stories that eventually the brakes will fail.

DOT 5 is all I will ever use in my restorations or cars where the complete brake system components have been replaced.

 

[73Coupe]

...snip

I would not, however, just flush out the system and then install the DOT 5 fluid, as I have heard other stories that eventually the brakes will fail.

DOT 5 is all I will ever use in my restorations or cars where the complete brake system components have been replaced.

I have heard that one must completely clean out the system in such a case, not just flush but disassemble, clean and re-lube upon assembly with the new fluid. At that point might as well just replace all of the rubber components like cups, hoses, etc.

I don't have any experience with the dot 5 though, still running the dot 3 in our fleet, but flushing it every couple years so it stays clean.

 

[Davea Lux]

I have heard that one must completely clean out the system in such a case, not just flush but disassemble, clean and re-lube upon assembly with the new fluid. At that point might as well just replace all of the rubber components like cups, hoses, etc.

I don't have any experience with the dot 5 though, still running the dot 3 in our fleet, but flushing it every couple years so it stays clean.

Firs off, if the fluid went away with not obvious signs of leakage, you are probably losing fluid into the power brake unit.

Tried to drive my 1965 Fury for the first time the other day, and Yikes! Found I had no brakes! Don't know where the brake fluid goes when a car just sets up for quite a time.

I have had this car on silicone brake fluid, but a Tech school auto shop instructor had doubts this was the best bet. I have always been told silicone is more inert and less likely to absorb moisture from the brake cylinders. Should I use DOT 5 again, or go back to the car's original brake fluid?

Two things, firstly if you are losing fluid without and obvious sign of leakage, it is probably going out the back of the master cylinder into the power brake unit.
Secondly, Dot 5 is the best fluid currently available, your instructors objections not withstanding.
Steel brake lines do not have to be replaced when converting to DOT 5 unless there is evidence of corrosion. It is highly recommended that all rubber parts be replaced. Rubber parts absorb some Dot 3 and Dot 5. Rubber parts that have previously been exposed to DOT 3 will tend to swell when they come in contact with DOT 5. Rubber parts will retain enough DOT 3 to still cause corrosion problems down the road even after converting to DOT 5.

I would suggest that you replace all of the brake hoses, wheel cylinders and the master cylinder. Remove the power brake unit and the input valve from the power brake unit. If you can get any liquid to run out of the power brake unit, it needs to be rebuilt. If you do all of these things, you system should be good for a very long time.

Dave

 

[Henrius]

Funny thing. I Googled "silicon brake fluid" and got tons of hits. The topic of DOT 3 versus DOT 5 is as controversial as abortion. There are people who swear one or the other has either ruined their brake components or made them last forever. The only agreement seems to be that silicone fluid holds air bubbles more easily and the brake pedal does not feel as hard. I concur with the observation from my experience.

 

[Davea Lux]

Funny thing. I Googled "silicon brake fluid" and got tons of hits. The topic of DOT 3 versus DOT 5 is as controversial as abortion. There are people who swear one or the other has either ruined their brake components or made them last forever. The only agreement seems to be that silicone fluid holds air bubbles more easily and the brake pedal does not feel as hard. I concur with the observation from my experience.

Dot 5 has a higher viscosity, and that sometimes makes it harder to get air bubbles out. Again, with proper bleeding and and proper installation as noted above, it will be good for a very long time. Properly installed, all corrosive issues are eliminated as nothing in the fluid is corrosive., but you still can not leave the vehicle outside or on a dirt floor and not expect problems.

Dave

 

[cantflip]

And for those of us who are not planning to rebuild/replace our entire hydraulic systems... just flush the DOT 3 with a high quality DOT 3 every couple years and you will avoid the majority of the problems associated with this fluid. The downfall is lack of maintenance, regardless of which fluid you use.

DOT 3, 4 and 5.1 absorb moisture. The "wet boiling point" comes into play with old fluid or fluid that was poorly handled (gravity bleeding). DOT 5 does not absorb moisture, which will condense and find it's way into the lower parts of the system... if excessive and neglected, corrosion can still happen.

You allow moisture into the system every time you open the cover to check the fluid. I don't hate DOT 5, and I'd seriously consider it for a system I was rebuilding completely. If you managed to get enough moisture into DOT 5, you could have trouble with the water boiling... but it isn't intended to be a lifetime fluid either and should be flushed/replaced as well. With a seldom driven car, there may be some ability to extend the interval with DOT 5.

Regardless of which glycol fluid you buy (3, 4 or 5.1) The DOT rating refers to MINIMUM standards for boiling point (the rest relates to chemistry/engineering stuff I don't concern myself with) There are DOT 3 fluids, like Ford's heavy duty version (IDK if the same stuff is still available at 550/284 dry/wet) that exceed the DOT 5.1 and 5 fluid minimum boiling points. Be careful about the part store bottle, you will find they may only meet minimum specs regardless of brand name. Don't buy stuff not recommended for street use, it may be lacking additives which allow longer service life... sports car racers change fluid often.

 

[detmatt]

I went from the 3 to 5 in my Imp last year or the year before and wish I would’ve in my vert especially after dribbling some dot 3 on the fresh paint of the inner fender...

 

[cantflip]

I went from the 3 to 5 in my Imp last year or the year before and wish I would’ve in my vert especially after dribbling some dot 3 on the fresh paint of the inner fender...

That is the primary reason I would consider the change... but not worth it IMO unless you're rebuilding the entire system.