i have converted over 100 cars from DOT 3 &4 to DOT 5 over the last 30+ years without problems. i am still no expert on the subject.


as far as the comments about DOT 5 uses. in my OPINION it basically a niche product usually reserved for the resto cars where they don't want the paint messes up.

there are much better brake fluids designed for racing with high boiling points than DOT 5 that are probably cheaper also.
as far as OEM using it. the OEM want cheap and easy to use over most other things. DOT 5 doesn't have either of those qualities.

my comments about it not causing problems are directed at residual amounts of the old causing problems with the new. never seen a problem with this. now mix a larger amount of the 2 in a car you are asking for troubles.
in my OPINION people who are commenting about negative comments about mixing the 2 are confusing the 2 and thinking they are the same thing.

as far as the water trapped in the brake line freezing causing problems. i THINK the freezing part will just force fluid thru the lines like pushing the pedal down before it will crack a brake line open.

When I worked at the Hyster dealership we had a new model come out that used DOT 5. Hyster sent out a service bulletin stating it was acceptable to use DOT 3 to add to or top off DOT 5. They claimed that even though it looked bad when it coagulated into goo, their research found no affect on performance.

That being said, I don't mix them.

When you do a change over, how are you bleeding it?

i do it the simple and easy way.
i remove as much of the old fluid in the master as i can.
i make a speed bleeder. i use a clear bottle, old soda plastic soda bottles work great, with a little of the NEW fluid at the bottom and a hose connected to the bleeder. make sure the other end is submerged in the fluid. MAKE SURE THE HOSE CONNECTS SECURELY TO THE BLEEDER. there can't be any air leakage at this connection. i usually make a hanger for the bottle with a coat hanger if i an doing this on a hoist, witch is most of the times i did this. i drill a hole in the cap to help secure the hose so it doesn't move above the fluid level.
attach the hose to the bleeder then open it. i start on the RR
them pump the pedal about 4-8 times. depending on the size of the master.
refill the master and pump another 4-8 times. DO NOT LET THE MASTER EMPTY OUT WHEN BLEEDING. this will create problems and you will have to run fluid thru the entire line to get the air you just added to the brake line.
repeat until you see the DOT 5. then move on to the LR, RF, LF.
then do the bleed again but this time pump about 4-6oz of fluid out each wheel.
check the pedal hardness. 8 out of 10 times you will be done.
you will use at least 1QT of fluid when you do this.

i like to attach a vibration device to the brake line being bled to help remove hidden air pockets. this is really effective on the rear lines. this also helps when bleeding the front calipers.

i also like to use the proper speed bleeders on the front calipers if they are available. these work a little better than the hose and bottle speed bleeder method.
as i posted before. PUMP SLOWLY. use slow and steady strokes with the pedal when bleeding.

auto part stores usually sell the same basic set up i make myself. black vacuum lines are OK to use. clear lines work a little better they let you see if any air enters the system..


i haven't done this job in about 10 years now that i an into newer cars with ABS.

Thank you.

I’m considering the dot5 since I have a limited use, cruise only car, with manual drums and nice paint. Seems I need to be very careful on the extra bleeding time and given my time this spring I’ll probably go with dot3 and just be extra careful. Won’t need to take out much as the fluid in the master is very clean and my usual spring routine is just to see some clean come out the clear hose. The change is obvious and doing every spring has served me well on many cars.

I used to run dot4, but I’ve read that one is more sensitive on absorbing moisture than dot3. Maybe a bunch of rags and a steady hand and hope for the best. What I saw makes me think either would be fine with my routine bleeding s keeping the water drained off anyway. Car will never be pushed that hard to test boiling point.

It’s a bummer somebody dint just put the dot5 in when they redid the brakes. It has new lines and hoses and even got a new master. Would’ve been so easy to add the purple and be done at that time.

DOT 3&4 are pthalate esters. DOT is silicone. I would never mix the two. The car won't blow up, but it's your brakes, so don't half-ass it IMO

i am no expert. but my basic understanding of brake fluids says the higher the boiling point of the fluid the more sensitive it is to moisture.

Curiously I emailed Eberg last January on the same subject, specifically on my Dot 5. system , because I had the master cylinder rebuilt and those guys told me that if I use Dot 5 their warranty is void. At that point I asked for an answer, but never got one. After reading what he wrote on Mopar Action I thought I had the solution, I.e. mix different fluids, but now things are more confusing than ever. By the way, I would not consider him a simple journalist, but an expert that has been writing advice for many years and is still going strong. So let's see what you now have to say. Tnanx.

I converted my '72 vette over to silicone 40 years ago after having the calipers stainless sleeved by SSBC due to pitting in the bores.
I haven't had any problems in all that time. The brakes will haul the car down from 145 mph at Talladega no problem.
The only place I wouldn't use silicone is on a race road course with repeated braking and heat buildup. I would use a racing brake fluid in that environment.
When I get the 'cuda back on the road, I'll be going with silicone. It's worry free, doesn't absorb water or affect the paint.

That’s the gist of what I read

I do want to thank everyone that chimed in. I find this the most bizarre car topic I’ve encountered. It’s almost universal that people love the stuff and have no issues or they hate it and it’s a carny rouse it’s so bad to deal with. I can’t think of another matter so hot and cold. I’ve seen everything from just put in what little you need to full flush to change all seals, it seeps out switches, change all rubber parts, etc.
Just a weird weird deal.

Now you can see why I just love this topic.

I don't hate it. All clean, fresh parts, street car, nice paint you're afraid to get messed up...maybe an OK option.

But it doesn't absorb water, which means moisture can puddle and corrode in the system. And it is compressible. And it can be hard to bleed. And mixing it is asking for trouble.

Race cars, I'm using a good, synthetic DOT 4, and I'm maintaining/flushing the system through once in awhile, keeping the pads fresh, resealing calipers when they need it.

The compressibility on some cars comes from long flexible lines, and not the fluid itself. And, whether braided or not, all flex lines expand.

dot 3 is cheap up front;...until the rot sits in on the tail in. dot 3 keeps the parts people in business. i've never owned a mopar that used dot 3 that didn't have the master cylinder rot and leakage. ever see a firewall on an older mopar that still had paint on it below the master cylinder? i took a chance with dot 5 27yrs ago and glad i did. i remember some years back on this website where this issue was brought up and the claim that dot 5 screws up the rubber parts in the system. i got to thinking that i had dot 5 in my car for maybe 15yrs so i'll take the system apart and look for damage. there wasn't any hint of damage to the rubber parts and the cast iron master and wheel cylinders were like new. can't say that for dot 3. how long does it take for dot 3 to make all the cast iron parts look like a cat crapped in them?

Good topic...best to read up on this stuff, and get as many points of view as possible.

Having previously done so, I saved two such documents. I think they do a fair job of objectively laying out the details w/o attempting to sell a product.

Here they are:

1) Brake Fluid
2) Brake Fluid Basics

...and I suppose if one wanted to stay with the Glycol based mix but derive some of the benefits of DOT5 performance, you could always pursue the DOT5.1 stuff....

My own personal experience

Ive built 4 maybe 5 cars using dot5 . NONE have had an issue post assy, ,,,,,,,not one.
These cars were all built from new components

From new over the counter master cyls and brake calipers to new lines, etc etc etc.


A friend of mine has a 1955 Chrysler 300, he had his master cylinder rebuilt by White Post. . Flushed the system before the install and switched to dot 5 .
Within a year he had a master cyl leak. Sent it back to WP . they rebuilt no questions asked.

Less than 6 months later it leaked again. This time WP asked what type of fluid. And when he told them DOT5 .

They said flush it and refill with dot 3 or warranty was void. So he did.

And he has had zero issues since, and this was maybe 6 or 7 years ago now

I can think of another topic that is just as bizarre. Ballast resistor. Do you need it, or not on a after market ignition, or coil. Oh boy. here we go. LOL

That's what I always thought:more prone to minor seepage/leakage. Like around seals or flare fittings.
But like exhaust systems: some moisture at seams aides to seal them in time.

I don't know what you are doing wrong, but I have never seen the issues you mentioned. Well, maybe I do know. How often did you flush your DOT3 brake fluid? Never? I just flushed the system in my 51 yesterday, the old fluid was just a bit darker than the new fluid, no crud, inside of the master looked new.





This issue with compressibility is primarily due to air in the fluid, which silicone can be more susceptible to. That's the air entrainment I mentioned.