switching from DOT 5 to DOT 4 brake fluid

jesusgatos · Dec 27, 2009

I just replaced all the wheel cylinders in mah deuce, and I would like to switch from DOT 5 to DOT 4. I bought/built a pressure-bleeder and a gallon of DOT 4 brake fluid this morning, but I just read about DOT 4 and 5 fluid mixing and turning to jelly. I was planning on using the pressure-bleeder to push air through all the lines, and then refill with fresh DOT 4 fluid. Is that good enough? I guess I could remove the lines from the wheel cylinders and blow-out the lines before the fluid goes through the wheel cylinders (installed, but they're all still dry). Guess the best way to do that might be to cap off 5 of the lines at a time (leaving one open), right? Any other advice? Should I flush the lines out with any other type of fluid first?

OldDominionIron · Dec 27, 2009

My question is why would you change from DOT 5 to DOT 4? DOT 5 is what the government uses in its vehicles. I'll be honest and say I'm not sure what the difference is, except for DOT 5's resistance to moisture absorption, which is greater than DOT 3. With the possible gelling of the mixture of brake fluids, I would say flush with air, DOT 4, air, then fill a final time with DOT 4.

Terexts · Dec 27, 2009

I believe the DOT 5 fluid is a silicone based fluid, where as the DOT 4 is polyethylene glycol based. The DOT 5 has a higher boiling point than the DOT 3 and 4 fluids. Don't know if that info helps or not, but its good to know i suppose.

jesusgatos · Dec 27, 2009

I'd rather not turn this into a DOT 5 vs. DOT 3/4 thread, but I'm switching to DOT 4 because I don't want water collecting in my wheel cylinders, and DOT 3/4 is widely available and much less expensive than DOT 5 brake fluid. Personally, I don't see ANY advantage to DOT 5 but that's not what I'm interested in talking about here. I just want to know if I'm going to create a dangerous situation by mixing DOT 4 fluid with any small amount of DOT 5 fluid that might be left in my braking system after I flush and refill.

doghead · Dec 27, 2009

For the best answer, you might contact a brake fluid manufacturer, and ask them.

glcaines · Dec 27, 2009

I don't know about problems mixing DOT 4 and DOT 5, but I do know that DOT 4 attracts water just like DOT 3. DOT 5 does not attract water as it is silicone based. DOT 5 is far superior to DOT 3 or DOT 4, albeit more expensive. I would recommend sticking with DOT 5 and thereby avoiding any potential mixing problems.

papercu · Dec 27, 2009

Just do it as a normal flush and fill, there is a TB or TM that tell you how to change the DOT-3 to DOT-5, 5 to 3/4 would be done the same way. Wayne

Blood_of_Tyrants · Dec 27, 2009

I would get a gallon of some kind of solvent like acrylic lacquer thinner or naphthalene and use it to flush the lines between the DOT 5 and DOT 4.

cbvet · Dec 27, 2009

If you really want to flush the system, use denatured alcohol.

NDT · Dec 27, 2009

Blood_of_Tyrants said:
I would get a gallon of some kind of solvent like acrylic lacquer thinner or naphthalene and use it to flush the lines between the DOT 5 and DOT 4.

I did the lacquer thinner flush when I went from Dot 3 to Dot 5 on my M135. That was 27 years ago (1983) and I have not needed to work on the brakes since.

jesusgatos · Dec 27, 2009

glcaines said:
I don't know about problems mixing DOT 4 and DOT 5, but I do know that DOT 4 attracts water just like DOT 3. DOT 5 does not attract water as it is silicone based. DOT 5 is far superior to DOT 3 or DOT 4, albeit more expensive. I would recommend sticking with DOT 5 and thereby avoiding any potential mixing problems.

Jeez guys, I'm really not trying to argue this point. Water is going to be suspended in DOT 3/4 fluid, and I think that's a good thing. Especially compared to what happens to water in DOT 5 fluid (collects at the lowest points). Personally, I'd just rather run DOT 4.

I did a few google searches and read that lacquer thinner is bad for the seals and rubber components. Any of you know if there might be a better fluid to use? I don't know anything about naphthalene.

Does anyone have a link to the TM that papercu is referring to?

papercu · Dec 27, 2009

It's TB 43-0002-87 if it is not in the Resource section, send a email address and I'll see if I can send you a copy. Wayne

stumps · Dec 27, 2009

jesusgatos said:
Jeez guys, I'm really not trying to argue this point. Water is going to be suspended in DOT 3/4 fluid, and I think that's a good thing. Especially compared to what happens to water in DOT 5 fluid (collects at the lowest points). Personally, I'd just rather run DOT 4.

I did a few google searches and read that lacquer thinner is bad for the seals and rubber components. Any of you know if there might be a better fluid to use? I don't know anything about naphthalene.

Does anyone have a link to the TM that papercu is referring to?

DOT 3/4 is sold and used in an anhydrous form. That means it is devoid of any water. When it is exposed to normally humid air, or water, the water goes into solution almost instantly. That converts the DOT 3/4 into a water based solution that is corrosive to cast iron parts, such as the wheel cylinders and the master cylinder. The corrosive action is evidenced by the fluid turning brown.

DOT 5 is not miscible in water. Not at all. It will not draw water out of the air like DOT 3/4. Any water that happens to drip into your master cylinder while you have the cover off will pool in the pockets that are specifically placed in the bottom of the reservoir to collect water and debris, and will remain there and probably form a little rust. But! There is no reason for any water to get there in the first place.

The military switched to DOT 5 because it was having problems with DOT 3 fluid literally sucking water out of the air at the master cylinder's vent, and becoming corrosive. This caused the need for a continuous cycle of brake service on vehicles that were mostly just parked on the macadam somewhere waiting for the Russians to invade. Kind of like the way we use our MV's waiting for the zombie apocalypse.

DOT 5 is a lot better suited to the kind of use most of us have for our MV's.

Before I switched the brake fluid in my "antique" cars to DOT 5, I used to always find the brakes dead when I got around to taking one out for a drive. Since I switched to DOT 5, I never have that problem. The problem now is usually something to do with the carburettors, or battery....

If you are worried about the expense, check out Spruce Mountain Surplus. They always have gallon cans of new DOT 5 for $40 shipped.

-Chuck

Blood_of_Tyrants · Dec 27, 2009

In the Navy we used naphthalene to clean acrylic. It is a fairly benign solvent. Most any solvent is tough on rubber parts and even denatured (also known as wood or methyl) alcohol will attack some rubber parts. The deal is not to allow the solvent to stay in contact with the rubber for too long. Push out s much DOT 5 ad you can with air then flush with solvent. Then use pressurized air to push out as much solvent as possible. The tiny bit of solvent left will not be enough to attack your rubber parts.

Ethyl (drinking) alcohol or isopropyl (rubbing) alcohol are probably two of the most rubber friendly solvents.

jesusgatos · Dec 27, 2009

papercu said:
It's TB 43-0002-87 if it is not in the Resource section, send a email address and I'll see if I can send you a copy. Wayne

here's a link

ericm38 · Dec 27, 2009

If your brakes do not leak there is not a lot of $ to stay dot 5. After you buy brake cleaner, or what ever to clean the lines. You would be about the same in $. I put dot 5 in all of my jeeps, you can get it on the paint, and it will not rust if you go in water or mud . My thought good luck . PS brake cleaner is what I would use and air.

stumps · Dec 27, 2009

jesusgatos said:
I did a few google searches and read that lacquer thinner is bad for the seals and rubber components. Any of you know if there might be a better fluid to use? I don't know anything about naphthalene.
?

Two definitely safe fluids for brake systems are brake fluid, and isopropyl alcohol.

Silicone DOT5 fluid completely ignores DOT3/4. They are not at all miscible. They are like oil and water. If you really want to be a contrarian, and put your system back to the inferior glycol based fluid, use DOT 3 to flush, and then just use DOT 3. There is absolutely no good reason to waste your money on DOT 4. It has nothing to offer a drum based brake system.

-Chuck

jesusgatos · Dec 27, 2009

Thanks for the input stumps (and everyone else), but let's put this to rest.

I'm going to install a Canton Racing 2-quart remote reservoir that I'll be be able to pressurize. That should make flushing the brakes a no-hassle affair. I'm literally putting EVERYTHING that I have/am into this vehicle (money/time/effort). I'm planning on living/traveling in it full-time for the foreseeable future, so I'm not trying to take any shortcuts. After reading a bunch about the trade-offs between DOT 4 and DOT 5 fluid, I've decided to go with DOT 4.

I understand what you guys are saying about DOT 3/4 fluid attracting water, and I understand that DOT 5 doesn't. But I don't like the idea of water collecting at the low-points, like wheel cylinders, where the water might cause corrosion and/or boil and cause the brakes to fail. In all likelihood, brake fluid will never be left in this vehicle long enough to cause damage to any parts, which brings me to my next point. I need to be prepared to service this vehicle, and I'm planning on taking it to many places where I imagine I might have a hard(er) time finding DOT 5 fluid. So if I can adapt this vehicle to use more widely available parts/fluids, I will.

I'm not trying to tell any of you that YOU should run DOT 4 fluid. Please feel free to continue this discussion, but I didn't start this thread asking 'What brake fluid should I use?'...

jesusgatos · Dec 27, 2009

stumps said:
Two definitely safe fluids for brake systems are brake fluid, and isopropyl alcohol.

Silicone DOT5 fluid completely ignores DOT3/4. They are not at all miscible. They are like oil and water. If you really want to be a contrarian, and put your system back to the inferior glycol based fluid, use DOT 3 to flush, and then just use DOT 3. There is absolutely no good reason to waste your money on DOT 4. It has nothing to offer a drum based brake system.

-Chuck

So you think it's be OK to just flush a bunch of DOT 3 fluid through the system? I'm just wondering why people talk about this jelly-sludge that mixing DOT 3/4 and DOT 5 fluid supposedly makes. I was only concerned that I might be making bigger problems for myself, and wanted to know how I should go about switching from one fluid to the other.

Reading this TM right now...

stumps · Dec 27, 2009

I understand what you are trying to do, but you need to know that water in DOT3 or DOT4 fluid will boil too!

I have read the trade-offs posted about DOT5, and in my opinion, they are all theoretical. They are posted by people that are trying to stir the pot for reasons known only to themselves. The only way water has to get into your system is the master cylinder. Unless you let about a teaspoon of water into the reservoir, it won't go any further.

Ten years down the road with DOT5, and the wear in your hydraulic brake system parts will be un-noticeable. Ten years down the road with DOT3/4, and you will be into your first new master cylinder. Fifteen years and you will have several bad wheel cylinders. I have done this experiment over and over again. I won't do it any further. DOT 5 is mecca for the antique car enthusiast.

-Chuck

switching from DOT 5 to DOT 4 brake fluid

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