Brake fluid -how to tell which kind you have DOT 3 or DOT 5

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OPCOM

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The important part first, the story is after.

DOT 3, 4, and 5.1 can be mixed if necessary, according to the instructions on the cans. The temperature range is higher as it progresses from DOT 3 to 4 to 5.1

DOT 5 and DOT 5.1 are not the same.

DOT 5 is a silicone based fluid, and DOT 3, 4, and 5.1 are glycol? based.

don't guess.... this is how to check:-

Essentially, there are two types. The first one is regular auto brake fluid. The second type is Silicone-based brake fluid.

You can not mix the two types of fluid (well, you can, but you will end up with a jelly-like mess).

You need to find out which you have already in the system. Here's how:-

1. Get a dropper (an eye or ear-dropper) and withdraw some fluid from the master cylinder.
2. Put it into a glass jar and then add some clean water to the fluid.
3. Now put the lid on the jar and shake it well.
4. Let it stand for a few minutes.
5. If the water and the fluid have mixed thoroughly and can not be separately identified, then you have regular auto brake fluid.
6. However, if the water and the fluid have not mixed, or have formed blobs or layers, then the fluid is Silicone.

That's all there is to it. Lots of people think that you can recognise which is which by looking at the colour of the fluid, or by its odor. Don't risk it. Use the test outlined above. It's already been said that DOT 5 is not always purple, or that it can change color after time to a yellow that looks like DOT 3.

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Being faced with this today, I dug the above info up from the mil veh list. It's worth posting here since a mistake can be costly and dangerous. My DOT 5 had indeed turned yellow, or maybe it was the original military fluid, which has been reported to be light-colored.

The M35's brake pedal had been requiring much more travel before engaging the brakes. Not finding any leaks but having put alot of miles on the truck, I checked the master cylinder and found it nearly empty! This was a disaster waiting to happen. The brakes had apparently worked just fine until the master was almost completely out of fluid and then there was little warning before the brake pedal gained about 3" extra travel.. The truck still stopped OK when hot but it didn't 'feel' right. When cold, the brakes had become very mushy. What was going on is that the fluid level had gotten low enough to where air was entering the master cylinder itself, creating the mushy feel. Inspection revealed that the resivoir was about dry. As I poured the new fluid in, I could hear air bubbling up from places air should not have been. It took almost the entire 12oz can. There was some concern that after refilling the master cylinder resivoir that the system would have to be bled. I was fortunate in that after gently and slowly working the pedal a number of times, the air trapped in the cylinder bubbled up and I have a nice hard pedal again. It may still need to be bled on general principles, first needs a road test.

The moral of the story I suppose is to check the fluid periodically, no matter what a pain in the butt it is, even if there seems to be nothing wrong. Several times I almost cursed the guy that decided to put the master cylinder right under a cab frame mount.

The vent line came off with a 3/8" line wrench and the resivoir cap came off with a 11/16 wrench. The resivoir cap 'nut' was square so an open end wrench fit best.
 

devilman96

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RE: Brake fluid -how to tell which kind you have DOT 3 or DO

You can spare yourself a lot of agony by removing the vent cap and adding a barbed fitting and line up to the fire wall which will allow for a remote tank to be mounted making fluid checks much easier. There are some motorcycle reservoirs which are suited to the job, though small your just looking for a fill and level point to check as you already have the masters reservoir to hold the required amount of fluid. Many autos with remote power steering reservoirs are well suited to the task too.
 

73m819

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RE: Brake fluid -how to tell which kind you have DOT 3 or DO

wow, what a great idea, thanks, its funny how simple little things like this can make life so much easier yet goes unthought of for so long, thanks again for such a neet idea.
 

G744

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RE: Brake fluid -how to tell which kind you have DOT 3 or DO

I always check by getting a tiny bit on a finger and touching it to the tongue...3&4 are bitter, 5 is like olive oil. Rinse. Repeat if necessary. Chase with favorite beverage!

dg
 

hndrsonj

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so what would have to be changed to go from DOT 5 to DOT 3? I am guessing all rubberized components-wheel cyls, master cyl-ect?? As for this remote resevoir, anyone have any pics? If it's a remote resevoir, why couldn't a low light be hooked up?
 

Recovry4x4

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Do a complete and thorough flush. Empty the master cylinder with a baster if you can before you start. It needs to be a very thorough flush as you don't want both fluids in there. Any specific reason you're changing?
 

Wolf.Dose

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Dot brake fluid

If you change from DOT 3, 4 or better to DOT 5 silicone based brake fluid, you have, to avoid desasters, wash your brake system with concentrated alcohol. This means you have to take out all brake parts, strip to the last nut and bolt, wash in alkohol (including all tubes and hoses), regrease with brake grease blue, for ex. ATE, reasemble and reinstall. If you do it not this way, several rubber parts will fail. Then your brake fails.
Wolf
 
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CCATLETT1984

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If you change from DOT 3, 4 or better to DOT 5 silicone based brake fluid, you have, to avoid desasters, wash your brake system with concentrated alcohol. This means you have to take out all brake parts, strip to the last nut and bolt, wash in alkohol (including all tubes and hoses), regrease with brake grease blue, for ex. ATE, reasemble and reinstall. If you do it not this way, several rubber parts will fail. Then your brake fails.
Wolf
With our vehicles being DOT5 from the military, any changes go the other way around.
 

Wolf.Dose

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Brake fluid

Cleaning the brake system is recomaned for the other way around also. Do you know how long the fluid was in the vehicle. Was it installed properly? Was the old rig while beeing changed to silicone treated propperly. You do not know. I any Army / public service of this world there are somewere sometimes working idiots who ignore the roules. You and I do not want to suffer from them. So clean your system as I do it!
Wolf
 

roger-wilco-66

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Next to tasting the fluid and risking "Sgt. Half-Masts Revenge", you easily can check what brake fluid is in your system by mixing it. DOT3 and DOT5 do not mix. So if you put a little DOT5 in a jar and drain some fluid out of the system on top of it and it does nnot mix, you have DOT3 in your system.
 

AKJEEP

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Somewhere in here, it can be said that both types have their advantages and disadvantages.
the standard automotive brake fluid (DOT3 and the like) are very suseptable to moisture. given time, they will even absorb moisture through the cast iron master and wheel cylinders. if you do not thoroughly flush the system periodicly, it will be contaminated. if you live in a more humid climate, this occurs even quicker.
Silicone fluid (DOT5 for example) does not have this problem with water/moisture. that is why it is prefered for use where you are exposeing it to more water. But DOT3 brake fluid does not compress, where DOT5 will just a little bit. If you change over from DOT3 to DOT5, you will notice a 'spongey' feel to the pedal. You can drive yourself insane bleeding, and it will never get better. Nothing wrong with it - it's inherrent to the fluid. But some people just cannot get used to that feel under their foot
 

Gunnermac

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DOT 3 &4 are hygroscopic, they absorbe water. Will rust metal lines ,master cylinders and wheel cylinders. With a single compartment suicide brake system any leak can be a very bad thing. A rusty line can suddenly fail under braking. This can make for a very bad day for you and possibly others. I think that is why all the MV I have delt with have DOT5. There is no way I would use any thing else.
 

plym49

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Somewhere in here, it can be said that both types have their advantages and disadvantages.
the standard automotive brake fluid (DOT3 and the like) are very suseptable to moisture. given time, they will even absorb moisture through the cast iron master and wheel cylinders. if you do not thoroughly flush the system periodicly, it will be contaminated. if you live in a more humid climate, this occurs even quicker.
Silicone fluid (DOT5 for example) does not have this problem with water/moisture. that is why it is prefered for use where you are exposeing it to more water. But DOT3 brake fluid does not compress, where DOT5 will just a little bit. If you change over from DOT3 to DOT5, you will notice a 'spongey' feel to the pedal. You can drive yourself insane bleeding, and it will never get better. Nothing wrong with it - it's inherrent to the fluid. But some people just cannot get used to that feel under their foot
DOT 5 can harbor millions of tiny air bubbles and this is the reason for the spongy feel or, in some cases, an inability to bleed the system. (more pronounced in lower pressure, lower swept volume braking systems as on a motorcycle). Never shake DOT 5 as this introduces bubbles. Let the conatiner sit. Also, whenever possible, reverse power bleed a DOT 5 system so that you are pushing any bubbles up and out instead of down and inside. You will not see these bubbles - they are that small - but they are there. Done properly, a DOT 5 system will feel as firm as a DOT 3.

Topping off a DOT 5 system with DOT 3 leads to disaster. The two do not mix and form a jelly-like goo. Personally I do not trust any system where this has happened as it is extremely difficult to ensure that the cleaning/flushing is perfect. It is a very expensive mistake to make.
 

acesneights1

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I'm concerned about making a mistake here. Anyone know where I can send out a sample of it to determine what it is ?
The master looked newer so the system has been worked on.
 
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