Dot 4 or Dot 5 brake fluid?

SSsssteamer

SSsssteamer

Well-known member
Wanting a brake fluid that will last forever, after a complete brake rebuild on a non functioning system, I put the Dot 5 Silicone brake fluid into, my 1957 Skyliner that has power brakes. I did the same thing to my 1916 Stanley Mountain Wagon. On the Stanley I installed disc brakes to the rear axle. I have about the same problem with the both of them. The brake pedal is not firm but feels soft. The brakes do work but work poorly. The Skyliner's brakes feel like standard brakes instead of the "Swift and Sure" brakes that they are. I have trouble shooted the system several times as well as going through every component. The Stanley has had the same detailed attention. Is the problem with using the silicon Dot 5 brake fluid instead of the old Dot 4 fluid? Has anyone else had this problem when they had changed out to Dot 5 Silicone fluid?
 
Hi SSsssteamer,

While I don't have direct experience with this, I researched it in detail a few months ago, and traded notes with some friends who've used DOT 5. Overall the info I found on DOT 5 fluid usually described the same symptoms you describe. The cause is that the Silicon fluid is more compressible than DOT 3/4, so the result is more travel in the pedal as the fluid absorbs the pressure.

Here is a link to a site with some research on this.

Hope this helps a bit, interested to hear if anyone on the barn has practical experience with this.
 
I have had trouble bleeding DOT 5. It seems little bubbles are hard to clear out. Let it sit a week and try again. Maybe rock the car on its springs a bit before trying. If you live in a cold climate, the results are worth the effort.
 
Dot 5 will not give you the same pressure feel as dot 3/4. It has the advantage of no moisture, so you will not get rust inside the lines or cylinders when sitting for long periods of time, a lot of people even have to change the pressure fed brake light switch to either a motor cycle switch or even go to an electric switch on the pedal. I did the motor cycle switch and then had to make a bracket and mount a switch on my pedal also. My brakes will lock up and stop the car but just have a sloppy feel. I work on cars for a living and have for 35 years, if you decide to go back to dot 3/4 you are going to have a h**l of a time getting all of the 5 out of the system, they will not mix !!! Get it very clean and go back to 3/4 and just bleed the system clean every couple of years and you will not have a moisture problem. Ken.
 
I've used DOT 5 in a '57 Ranchero with manual brakes and it was perfect. No bleeding problems, good pedal, normal stopping.

I built a street rod with Mustang two front brakes, ford 9" rear, manual Mustang master cylinder under the floor. It has always had a spongy pedal, but not excessive travel. I've been driving it for 19 years this way, so I guess it isn't TOO big a problem. I can't compare pedal pressure to DOT 3 or 4 because I never ran it.

So I guess the success is just as you have found: IFFY.
 
Drove this car from 1972 till 1992 with the conventional hydraulic fluid. In 1992 when I had the body off the frame, I decided to replace all brake lines and components and change to silicone fluid (DOT5).
Today. the wheel cylinders, master cyl and brake lines still look as good as the day I installed them (not a spec of rust). Thats the upside.
The downside is, the brakes do feel more "spongey" and there is more pedal travel to stop compared to the old DOT3 fluid.
Other than cleaning the front brake shoes annually, I have not done any service on any of the brake system components since I installed the system in 1992. Thats over 20 years I have not spent one red cent on these brakes with the DOT5 silicone fluid. And for a car that runs upward of 60 miles a day, 5 days a week or more, I figure thats pretty good.
Had I kept the conventional system all these years, I would probably have had rebuilt the wheel cylinders twice and replaced the master cylinder at least once.
My 12 year old niece wants me to give my car to her. Her mother (my sister) hates the car because it is old and does not have seat belts and has never allowed my niece to go near the car, let alone ride in it. You just gotta laugh! Funny thing is, my sister drove this car everyday for about three years back in the early 1980's before she got her first car.
 
Great information. So I read that I am not alone with the spongy brake pedal while using Silicone Dot 5. We have fifteen cars in our collection and the only two with brake problems are the ones that are using the Silicone Dot 5 brake fluid. With our Stanley, I have been trying to get a firm brake pedal for three years now. The Skyliner has been a continuous fight for the last two years. With our Skyliner, by the time the soft brake pedal has been pushed to engagement, the "Swift and Sure" power brake assist has been used up. Beyond that power brake assist travel distance, it is a manual pedal. Looks like my best choice is to clean the systems out of their silicone fluid, and go back to the old Dot 4 brake fluid. Darn. And I thought that I had only used the best brake fluid for these cars. Thank you.
 
One thing to look out for when using Dot 5 is that IF (and I'm not suggesting this is your issue) any moisture gets into the system it is not absorbed by the brake fluid as it is when using Dot 3 and 4. The moisture settles at the lowest point in the system (wheel cylinders) and will boil at 212°F, causing "spongy brakes" or loss of brakes altogether.

You may want to look into Dot 5.1, a non-silicon version of Dot 5.
 
I use Dot 5 in several vehicles with good results. No spongy pedals here but I do replace the flexible brake lines regularly. The oem flexible lines do get soft with age which in turn gives a 'soft' pedal feel.
 
I am running DOT 5 in my 56 T-Bird without any problems ...this year will be two years...so far so good....I like the fact that it does not destroy your paint if you spill some on the engine or firewall...J in CT
 
I've run Dot5 in my 56 f100 since 1984 and haven't had the first problem. Changed the brake shoes a couple years ago and pulled the rubber boots on the ends of the wheel cylinders and there were no signs of any leaks. Also have Dot 5 in my 55 willys and no problems there either.
 
I found another advantage to silicon brake fluid. I recently sold a 39 Ford ton and a half truck. I told the new owner that it needed a wheel cylinder replaced as it was leaking. He told me if I put silicon brake fluid in the system it will swell the rubber and stop the leak. He came out bought the truck put the silicon brake fluid in and drove it to San Francisco about 25 miles from my house. He called me when he arrived and said he had no problems at all. I asked about the brakes and he said they were fine. I have spoke to him several times since then and all is well.
 
whizzernick said:
I found another advantage to silicon brake fluid. I recently sold a 39 Ford ton and a half truck. I told the new owner that it needed a wheel cylinder replaced as it was leaking. He told me if I put silicon brake fluid in the system it will swell the rubber and stop the leak. He came out bought the truck put the silicon brake fluid in and drove it to San Francisco about 25 miles from my house. He called me when he arrived and said he had no problems at all. I asked about the brakes and he said they were fine. I have spoke to him several times since then and all is well.
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Are you saying that he added some Dot 5 to the regular brake fluid (Dot 3 or 4) that
was already in the brake system ?
Dot 5 (silicon) should NEVER be mixed with Dot 3 or 4.


Are you sure he didn't add Dot 5.1 which is NOT silicon, and the Dot 5.1
is compatable with Dot 3 or 4.












.
 
I used DOT 5 silicon in 1982 in my A sedan street rod, 90K over the years, Still in it with no leaks. Just instaled it my A V8 with 39 juice brakes. I did have troublie with leaking out the plunger of the master cylinder. a real hassle but after a fine hone it did quit leaking.
 
I have 4 old Fords with DOT for many years and have good firm pedal. I just finish a 36 with DOT 5 and have spongy pedal and have bled twice. Brake work fine once I get past the spongy part. I might bleed again.
 
A point that has not been mentioned is when you replace your brake shoes if the new shoes are not arc ground to be a perfect fit to the diameter of the brake drum then you are going to have spongy brakes. The reason this happens is that most old drums have been turned on a brake lathe and can be over by .050, .075 or even .100. If you lay your new brake shoe in the drum when it's off you will see that the shoes will not be a flush fit, thus back in the day they would be arc ground to match the contour of the over sized drum. This is almost a lost art as most of the old garages and machine shops have relegated these machines to the back of their shops or junked them. Today shops will tell you your drums are wore out and need to be replaced. When this happens you will most likely have a perfect fitting shoe as the new drum diameter is to stock specifications and the shoe has been manufactured to orig. specs. Drop the shoe in the new drum and you will see a flush fit thus no spongy brakes if all the air has been purged from the system.
 
On the use of DOT5 silicon, I was told that if you pump the brake pedal too quickly, it will create bubbles at the master cylinder, and a soft brake. When bleeding, pump very slowly. Once you create the bubbles, it takes a while for them to dissipate.

Milt K from Pa
 
I use DOT 5 in all my old Fords with no problems and the pedal is firm but Chuck at Chucks Trucks(an old Ford truck supplier in Ct) tells me that if the vacuum booster on power assisted brakes Sufis it into the engine that the engine is toast so I took it out of my F-7.
 
Last year a friend made the switch and had nothing but trouble. I had been considering it and did extensive research before just doing it. I found that, basically the comments of Y-Blockhead are right-on.

When silicone came out, it was the new greatest thing. Then the drawbacks and issues that arise when put in a vehicle that was not designed for it started to surface.

It is now recommended only for vehicles that came from the factory with it and for race track cars that need more than DOT4 heat resistance. I have a very late model Cadillac, 4 wheel independent suspension, aluminum frame, all the high tech - factory calls for DOT3. I have a brand new GMC truck - factory calls for DOT3.
 
In our '57 Skyliner, today I changed out the Dot 5 silicone fluid for the Dot 3 brake fluid. My power assist "Swift & Sure" brakes only work on the top two inches of the pedal travel. The Dot 5 silicone fluid took two inches of pedal travel before they would have full pedal pressure. Therefore, I didn't have any vacuum assist on the brakes at all. With the Dot 3 brake fluid, there is a good solid pedal at only an inch of pedal travel. That still leaves me another inch of vacuum assist power brakes to use if I need it. All I can say is "Holly Crap they work good!" The brakes work perfectly now. Thank you for helping me sort this out. The next car to do is the 1916 Stanley Mt. Wagon. I expect the same good results by going back to Dot 3 brake fluid.
 
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