Convert Brakes: DOT 5 --> DOT 4

[Bill6298]

Hello,

The previous owner of my car converted the brake system to DOT 5 silicone fluid in the late 1990s. I am about to overhaul the entire system including replacing the soft lines.

Has anyone converted back to DOT 4 before?

Reading about converting in the other direction suggests I need to flush the hard lines with denatured alcohol but I don't know if this applies in my case.

I'm not sure if I should continue with DOT 5 but I suspected this is what has always made the pedal feel spongy.

Appreciate any advice here.

Thanks

Bill

[David T]

Hello,

The previous owner of my car converted the brake system to DOT 5 silicone fluid in the late 1990s. I am about to overhaul the entire system including replacing the soft lines.

Has anyone converted back to DOT 4 before?

Reading about converting in the other direction suggests I need to flush the hard lines with denatured alcohol but I don't know if this applies in my case.

I'm not sure if I should continue with DOT 5 but I suspected this is what has always made the pedal feel spongy.

Appreciate any advice here.

Thanks

Bill

To "convert" back to DOT 4 you must disassemble the entire brake system and flush it all out with Alcohol. That means each line, each caliper, and the master cylinder. Yes, DOT 5 will give you a spongy brake pedal, the DOT 5 is compressable because you can never get all of the air bubbles out of it. If you have a 5-speed you should do the clutch system too. A lot of car owners went to DOT 5 because they thought that since it is not hygoscopic (does not absorb moisture) it would not rot the system. Did not work out so well. Besides the spongy brake pedal if moisture does get into the system it eventually collects in low spots and the system can rot. On old cars with drum brakes you can never get a stiff pedal so you can go all the way to the floor and still not get enough braking. DOT 5 was meant for racing because DOT 4 would boil (it has a lower boiling point than DOT 5). The best you can say about DOT 5 is that it won't damage your paint!

[Bill6298]

To "convert" back to DOT 4 you must disassemble the entire brake system and flush it all out with Alcohol. That means each line, each caliper, and the master cylinder.

Yes, let?s hope I don?t have many of those low spots where water will have separated and accumulated. I am hoping that I will not need to be replacing hard lines.

My plan is to start with some compressed air on the hard lines and then try to flush them with a large syringe.

[Helirich]

Hello,

The previous owner of my car converted the brake system to DOT 5 silicone fluid in the late 1990s. I am about to overhaul the entire system including replacing the soft lines.

Has anyone converted back to DOT 4 before?

Reading about converting in the other direction suggests I need to flush the hard lines with denatured alcohol but I don't know if this applies in my case.

I'm not sure if I should continue with DOT 5 but I suspected this is what has always made the pedal feel spongy.

Appreciate any advice here.

Thanks

Bill

So what is the fear if you don?t get every last drop out? Spongy brakes? You already have that.

I must confess, I don?t know anything about it. Maybe the mixture is like alien blood and melts through several decks of your space ship.

[Bill6298]

So what is the fear if you don?t get every last drop out? Spongy brakes? You already have that.

I must confess, I don?t know anything about it. Maybe the mixture is like alien blood and melts through several decks of your space ship.

They can gel.

[Josh]

Yes, let?s hope I don?t have many of those low spots where water will have separated and accumulated. I am hoping that I will not need to be replacing hard lines.

My plan is to start with some compressed air on the hard lines and then try to flush them with a large syringe.

I agree with what david said.

You will want to flush the line with more than just air, like he said alcohol or mineral spirits I have used in the past.
All rubber components should be replaced. So the master and slave cylinders, soft lines, and rebuild the calipers. This is the only way to make sure it is all gone.

[todd1561]

I guess I'll be the lone opposition here. I switched my car to DOT5 3 years ago after cleaning up the mess/damage corrosive DOT4 did to my frame epoxy when I had a master cylinder leak drip into the fuel tank holding area sometime in the past. The pedal feel with DOT5 is very stiff, no different than DOT4. Anyone that says it's spongy either has no actual experience with it and is just parroting what they've read or they didn?t bleed it properly. I use a Speedibleed pressure bleeder that works great. After a year of DOT5 in the brakes I switched my clutch a couple years ago and that's been great too. Very firm pedal and easily fully disengages the clutch with plenty of pedal travel left.

So since you?re already on 5 I?d just continue with it and see what success you have. Properly switching between the 2 is a lot of work.

[todd1561]

One other thing, I think the concern of "geling" when DOT4/5 are allowed to mix is a little overblown. I put equal amounts of each in a cup for probably close to a year and it looked the same at the end. The two didn't really mix, they just stayed separate more or less. When I switched I did flush the lines with denatured alcohol and was replacing the master and rebuilding the calipers so I followed the best practices anyway. But comments that suggest leaving even a trace in the system I think are being overly dramatic.

[Bill6298]

Going to have to clean the hard lines individually. There's no way to get any fluid through the system as-is between the condition of the master and likely collapsed brake hoses. Not the end of the world. I already have to do the same thing on my hard fuel lines. At this point, it's not really about getting DOT 5 out of the system and is about getting rid of any contamination given the car sat for so long.

Shop air only popped out 4 of the pistons. The rest caused my air chuck tip to blow off the nozzle. I did a quick 30 minutes in the ultrasonic cleaner but now everything is soaking in a de-greaser and evapo-rust. It's not clear if the pistons are pitted or not. I'll give it 24 hours.

719007190171902

[Bill6298]

Does anyone know if these are the original Gerling? I can't make out the writing but it appears to say British Pat.

7190371904

[Helirich]

One other thing, I think the concern of "geling" when DOT4/5 are allowed to mix is a little overblown. I put equal amounts of each in a cup for probably close to a year and it looked the same at the end. The two didn't really mix, they just stayed separate more or less. When I switched I did flush the lines with denatured alcohol and was replacing the master and rebuilding the calipers so I followed the best practices anyway. But comments that suggest leaving even a trace in the system I think are being overly dramatic.

That?s what I was thinking. Even if it gelled, I don?t really see it making the brakes fail catastrophic. Like you say, it doesn?t hurt to follow best practices.

[r00b]

If you are going to change dots, you might consider dot 3. Although dot 4 has a higher boiling temperature than dot 3, I realized I'm never in situations where that would matter. Driving in the mountains I downshift and hardly touch the brakes. Dot 4 absorbs moisture faster, you'll have to change the fluid less often with dot 3. If you're using your car as a race car then maybe go with dot 4, if not then dot 3 will be fine.

[Bill6298]

If you are going to change dots, you might consider dot 3. Although dot 4 has a higher boiling temperature than dot 3, I realized I'm never in situations where that would matter. Driving in the mountains I downshift and hardly touch the brakes. Dot 4 absorbs moisture faster, you'll have to change the fluid less often with dot 3. If you're using your car as a race car then maybe go with dot 4, if not then dot 3 will be fine.

Yeah I doubt I will need the higher boiling point. Dot 3 should work for me.

Ultimately the water absorption qualities of dot 5 did not save all of the pistons. Not blaming the fluid type. Length of time sitting is to blame. At $15 a pint and less availability, I don?t see the use case for this vehicle. I would rather change the fluid every two years.

71905

Some of them actually aren?t too bad on the face but the edge near the groove is pitted.