switching from DOT 5 to DOT 4 brake fluid

[jesusgatos]

Yeah, I know that even a little bit of water in DOT 3/4 fluid will lower the boiling point substantially. But water's going to get into anything that's vented to atmosphere, right? And if water is heavier/denser than DOT 5 fluid, where is it going to settle? And then what's it going to do? Instead of the water being suspended in the fluid (like it would be in DOT 3/4), wouldn't you imagine that it would be more likely to cause corrosion/rust when it settles at those low-points? And if those low-points are the wheel cylinders, we're talking about a much lower boiling point than even contaminated DOT 3/4 fluid, right? Please understand, I'm not trying to be argumentative. I appreciate your input and would like to know if any of the assumptions that I'm making (based on what I've read) are off-base. I can see why DOT 5 fluid is/was attractive to the military and antique car enthusiasts, but I'm building a truck that will be driven often and serviced regularly.

 

[stumps]

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...

If you want to do the job "right", you will have to disassemble every wheel cylinder and the master cylinder, blow out all the lines with clean dry oil free air, rinse the parts in isopropyl alcohol, and then flush with DOT 3, or DOT4... whichever you are going to use.

Then reassemble all of the wheel cylinders, and master cylinder and bleed the system.

I don't think it is necessary to be that "right".

DOT 5 is absolutely unmiscible with DOT3 or DOT4. They stay in their own separate puddles in the system. The result is ugly when it gets shook up, but like salad oil and vinegar, it settles back out with the DOT 5 on the top and the DOT 3/4 on the bottom.

If you simply flush with DOT3/4, wait a bit, and then do it again, the DOT5 will be displaced and will go out the bleed valves and be gone. The system will be back to the way it was when it was delivered by the manufacturer.

Hopefully, the DOT 3/4 won't activate a problem in the rubber that makes it swell from oily finger prints that got onto the rubber and cast iron parts when it was serviced at some point.

If you really want to be certain that you will always have brake fluid available for your military vehicle, use DOT3. It is available everywhere, and is exactly what was meant to be used in the system when it was originally designed.

I don't like to change things that are working nicely, so I would keep the system DOT 5, keep some on hand, and consider it yet another of the many parts for your truck that can't be bought at Rudy's Garage... Cold Beer and Welding To Go.... and if I got in a real pinch, and had to have fluid, I wouldn't hesitate to toss a can of DOT3 into the mix, and flush out the mess at some later point... just like Uncle Sam did. Every single one of our trucks lived through the change over between DOT3 and DOT5. In the interim period, DOT5 was routinely used to top off DOT3 systems until they could be reman'd to DOT5.

-Chuck

 

[stumps]

Yeah, I know that even a little bit of water in DOT 3/4 fluid will lower the boiling point substantially. But water's going to get into anything that's vented to atmosphere, right?

Not necessarily. There isn't any reason for there to be a wholesale transfer of air into and out of the master cylinder. The volume of air over the fluid is small, so it won't breath all that much, and the DOT5 won't seek out the humidity from the air like DOT3/4. The only way water is getting in is condensation, and I don't see that happening.

And if water is heavier/denser than DOT 5 fluid, where is it going to settle? And then what's it going to do? Instead of the water being suspended in the fluid (like it would be in DOT 3/4), wouldn't you imagine that it would be more likely to cause corrosion/rust when it settles at those low-points?

The first and biggest low point is the bottom of the master cylinder's sump. The master cylinder doesn't draw off of the bottom, so there the water will sit until you remove it. Water by itself doesn't cause rust. It needs absorbed air to make rust happen and air isn't absorbed in DOT5, so the water doesn't even cause rust. [all hot water boiler systems use bare iron/steel in direct contact with the heating water... no additives nothing: No air, no rust.]

Air, however, does get absorbed into DOT3/4 with water vapor. It will rust.

And if those low-points are the wheel cylinders, we're talking about a much lower boiling point than even contaminated DOT 3/4 fluid, right? Please understand, I'm not trying to be argumentative. I appreciate your input and would like to know if any of the assumptions that I'm making (based on what I've read) are off-base. I can see why DOT 5 fluid is/was attractive to the military and antique car enthusiasts, but I'm building a truck that will be driven often and serviced regularly.

Two of my DOT5 serviced cars were daily drivers several were not. The two daily drivers have had DOT5 in them for more than 16 years, and have zero signs of deterioration. The fluid is clean and purple, no leaks, nothing. The wheel cylinders were bled when shoes were replaced, and the fluid was clear and purple. I have never had a car go that long without needing a new master cylinder... and these already had more than 100,000 miles on the master cylinders before they were rebuilt and converted to DOT5. It is like their brake systems were frozen in time.

You have let a bunch of folks with unknown agendas get you worried about problems that just don't happen. Some folks just have to find fault with everything new that shows up. It makes them feel important I guess.

It's like the story of John Henry and his race with the newly invented steam hammer.

-Chuck

 

[jesusgatos]

If you want to do the job "right", you will have to disassemble every wheel cylinder and the master cylinder, blow out all the lines with clean dry oil free air, rinse the parts in isopropyl alcohol, and then flush with DOT 3, or DOT4... whichever you are going to use.

Then reassemble all of the wheel cylinders, and master cylinder and bleed the system.

Well I just installed new wheel cylinders (and they're still dry), so it wouldn't really be too difficult to do it 'right'.

I don't think it is necessary to be that "right".

DOT 5 is absolutely unmiscible with DOT3 or DOT4. They stay in their own separate puddles in the system. The result is ugly when it gets shook up, but like salad oil and vinegar, it settles back out with the DOT 5 on the top and the DOT 3/4 on the bottom.

...I would keep the system DOT 5, keep some on hand, and consider it yet another of the many parts for your truck that can't be bought at Rudy's Garage... Cold Beer and Welding To Go.... and if I got in a real pinch, and had to have fluid, I wouldn't hesitate to toss a can of DOT3 into the mix, and flush out the mess at some later point... just like Uncle Sam did. Every single one of our trucks lived through the change over between DOT3 and DOT5. In the interim period, DOT5 was routinely used to top off DOT3 systems until they could be reman'd to DOT5.

Whoa, you're absolutely right! At least, the TM I just read seems to agree with you. Hah. This is actually the main reason why I didn't want to use DOT 5 fluid. I didn't want to be stuck somewhere without the right brake fluid, and I thought that if I added DOT 3/4 to DOT 5 fluid, it would mix and create that awful jelly that I read about. But if I can top-off with DOT 3/4 in an emergency, that's good enough for me.

Not necessarily. There isn't any reason for there to be a wholesale transfer of air into and out of the master cylinder. The volume of air over the fluid is small, so it won't breath all that much, and the DOT5 won't seek out the humidity from the air like DOT3/4. The only way water is getting in is condensation, and I don't see that happening.

The first and biggest low point is the bottom of the master cylinder's sump. The master cylinder doesn't draw off of the bottom, so there the water will sit until you remove it. Water by itself doesn't cause rust. It needs absorbed air to make rust happen and air isn't absorbed in DOT5, so the water doesn't even cause rust. [all hot water boiler systems use bare iron/steel in direct contact with the heating water... no additives nothing: No air, no rust.

Thanks for explaining that about the master cylinder sump. That's really good to know. The issue of water in the brake fluid is much less of a concern to me now. I think you might have convinced me to stick with DOT 5 fluid. Thanks for talking/typing through this with my stumps.

 

[StreetbikeDepot]

It was mentioned that when DOT3/4 and DOT5 are mixed together and shaken that it gets ugly......well DOT 5 gets ugly on its own when shaken, you should never shake DOT 5 it aerates very easily and will trap air for a long time and thats why it is advised be left still for a long while before used.

 

[jesusgatos]

Oh, and just for the record: the TM specifically said NOT to flush the brake system with any type of solvents when switching from DOT 3/4 to DOT 5.

 

[Keith_J]

DOT 5 is not going to draw humidity from the air, period. Keeping the master cylinder full will reduce humidity issues as will a long vent line.

DOT4 isn't any better unless you do a complete flush every 2 years. Which is cheaper in the long run? DOT 5, by a long shot.

The ONLY way to flush from DOT 5 to 4 is by complete blow-down, then flushing with DOT 5.

Yes, water will drift down to the wheel cylinders regardless of what fluid. Using a fluid that does not absorb water is far better than having to change it every 2 years.

 

[StreetbikeDepot]

I'm a DOT 3/4 user myself and I simply bleed the system thoroughly when I switch over. Never had any issues.

 

[XM 2742]

The military probably had a similar discussion years ago and decided to go with DOT 5.....................there should be a report around somewhere that outlines the argument made by the engineers responsible for the change. In theory the military is slow to change and researches an issue to death. Why reinvent the wheel?

 

[jesusgatos]

Well ______ does _____, so that's obviously the right/best way to do it, right? Ugh. I understand what you're saying, but let's not give the military TOO much credit. There are a lot (a whole lot) of things that we are able to improve on. Almost any aspect of these vehicles can be made mo betta. If DOT 5 really is the best brake fluid to use, that's great. I want to know that. But I want to know why. But to say that it's the best brake fluid to use simply because the military uses it is, well, I dunno - it just makes me want to slap my forehead with the palm of my hand. I bet you're right about that report though, and I'd sure like to find a copy.

 

[XM 2742]

I would bet my last nickle on the fact that the discussion of the military engineers and those on this forum is similar.

The ability of DOT 5 to not absorb water was the over rideing factor in the change. The master cylinder design collecting a few drops of water is a heck of a lot better than an entire system collecting moisture as with the old fluid. Those trucks sat all over the world waiting to haul stuff between wars and to charge a battery and go was a lot quicker than haveing every 18 year old driver running into every other 18 year old driver getting out of the storage depot.

Sounds to me like the military made the right decision ........................but it is your truck so don't let me disuade you from returning to DOT 4.

Getting the report is a long shot.............but hey.............you can give it a try. Nothing top secret about brake fluid. The enemy knows what we use if they read this site....lol.

I am starting on an XM211 and I'll bet my stopping dollar on DOT 5.........the next guy can run peanut butter if he wants to.

 

[Keith_J]

Well ______ does _____, so that's obviously the right/best way to do it, right? Ugh. I understand what you're saying, but let's not give the military TOO much credit. There are a lot (a whole lot) of things that we are able to improve on. Almost any aspect of these vehicles can be made mo betta. If DOT 5 really is the best brake fluid to use, that's great. I want to know that. But I want to know why. But to say that it's the best brake fluid to use simply because the military uses it is, well, I dunno - it just makes me want to slap my forehead with the palm of my hand. I bet you're right about that report though, and I'd sure like to find a copy.

If you were to store/operate a vehicle in a constant temperature conditions, DOT 5 would never have an issue, EVER. Where DOT3/4 will eventually saturate with water from humidity if both have the same system.

The only time DOT 5 can get water vapor is when the temperature drops. During the summer, the dew point can be as high as 75 F. If the temperature drops to 45 with that air trapped in the master cylinder, there would be less than 2 milligrams of water now in the brake fluid IF there were 20 cubic inches of air in the master cylinder. This is such a slight amount it just doesn't matter, it will all disperse.

I have yet to find a corroded wheel cylinder on all the FMVs I have worked on. Now they do weep, this is a problem with vehicles that have not seen use. They all have responded to bleeding and solid brake pedal restored.

These are older vehicles, they need some TLC. Buy tools, learn how to deal with it and have fun.

 

[quarkz]

If you want to verify the gel-ing rumors, get some empirical results of your own.
Mix a little of the two of them together and see for yourself.
Then be a good guy and post up the results for the rest of us.
Heck while your at it add a little of the solvents mentioned and report those results as well.

 

[stumps]

If you were to store/operate a vehicle in a constant temperature conditions, DOT 5 would never have an issue, EVER. Where DOT3/4 will eventually saturate with water from humidity if both have the same system.

Yep!

The only time DOT 5 can get water vapor is when the temperature drops. During the summer, the dew point can be as high as 75 F. If the temperature drops to 45 with that air trapped in the master cylinder, there would be less than 2 milligrams of water now in the brake fluid IF there were 20 cubic inches of air in the master cylinder. This is such a slight amount it just doesn't matter, it will all disperse.

Most of the moisture that comes out of the air will settle on the sides of the master cylinder cap and reservoir. Some of it might drop into the brake fluid, but most of it will just hang around on the wall until it warms up again and goes back into the air.

I have yet to find a corroded wheel cylinder on all the FMVs I have worked on. Now they do weep, this is a problem with vehicles that have not seen use. They all have responded to bleeding and solid brake pedal restored.

The problem with a soft pedal appears to be the result of using the same bobble headed one-way vent on the master cylinder that they used on the gear cases. These vents allow passage of air out of, but not into the master cylinder. Given enough temperature changes, the master cylinder will have a mild vacuum in it, and that will draw the rubber piston cups back into the cylinders, and can draw air past the weakest cup into its wheel cylinder. I plan to replace my master cylinder's vent with a vent tube that goes into the air cleaner housing. I plan to do the same with all of the gear boxes, and the differentials too.

These are older vehicles, they need some TLC. Buy tools, learn how to deal with it and have fun.

One of the most fun things about MV's is you can fix these trucks yourself.

-Chuck

 

[ALFA2]

May be this will help, to actually get the Dot 5 and Dot 3/4 to gel together in the same system, and form strange, semi solid masses, you will need: lots of Heat, Pressure, a little moisture and some old Dot 3/4 about 30% by system volume. ( Less than 10% by volume of Dot 3/4 mix will simply not gel in your system, it just will not mix well with other fluids).Top of system with Dot 5, close cap, drive in a manner that would smoke the breaks, literally, until they faded, let cool till break function returns, repeat 3 more times, in close secession. Park the truck, let it cool completely overnight, try again in the morning, while cold, as in panic stop from 25/30MPH, stop, open cap, check content of the master cylinder, you should begin to see globules of matter, slimy and with some brown to it in color. That will get you gelling effect that has been advertised in many articles as a result of mixing the two or there types of break fluid.
​

To avoid this result, if you want to run DOT 4, just air flush the system, and for a more through clean, replace all wheel cylinders with new or rebuilt ones, as there is no true way to completely flush all of the contents of the wheel cylinders if they are contaminated with any liquid. With break lines disconnected from the wheel cylinder, after air flush, you can also do the denatured alcohol flush, and air again until dry. Then reconnect lines to the wheel cylinders, and proceed with filing the system with DOT 4, and bleeding as you were intending to do.

Hope it helps.

ALFA2

 

[stumps]

May be this will help, to actually get the Dot 5 and Dot 3/4 to gel together in the same system, and form strange, semi solid masses, you will need: lots of Heat, Pressure, a little moisture and some old Dot 3/4 about 30% by system volume. ( Less than 10% by volume of Dot 3/4 mix will simply not gel in your system, it just will not mix well with other fluids).Top of system with Dot 5, close cap, drive in a manner that would smoke the breaks, literally, until they faded, let cool till break function returns, repeat 3 more times, in close secession. Park the truck, let it cool completely overnight, try again in the morning, while cold, as in panic stop from 25/30MPH, stop, open cap, check content of the master cylinder, you should begin to see globules of matter, slimy and with some brown to it in color. That will get you gelling effect that has been advertised in many articles as a result of mixing the two or there types of break fluid.
​

To avoid this result, if you want to run DOT 4, just air flush the system, and for a more through clean, replace all wheel cylinders with new or rebuilt ones, as there is no true way to completely flush all of the contents of the wheel cylinders if they are contaminated with any liquid. With break lines disconnected from the wheel cylinder, after air flush, you can also do the denatured alcohol flush, and air again until dry. Then reconnect lines to the wheel cylinders, and proceed with filing the system with DOT 4, and bleeding as you were intending to do.

Hope it helps.

ALFA2

I'm sorry, but chemistry doesn't work that way. You can heat the wheel cylinders until they boil, and you still won't get any of the fluid that was in the wheel cylinders back into the master cylinder. It is just too far away. The heat from the wheel cylinders also won't conduct up the brake lines to the master cylinder, that is too far away as well.

...AND...

DOT5 and DOT3/4 are just too un-miscible. Within several minutes of their being thoroughly blended together, they will separate with the DOT5 on the top, and the DOT3/4 on the bottom. One thing that does happen, which I find quite interesting, is the DOT3/4 will wash the purple color out of the DOT5 fluid, and remain purple.

I have enclosed 3 pictures of a 50/50 mixture of DOT3 and DOT5 that I have in a jar. The first picture is with the fluids separated for a long time, the second has them stirred up, and the third is 3 minutes after they were stirred up.

Over the last 100 years of automotive history, we have all been told many stories that are absolutely known to be correct, but are in general, untrue.

Batteries discharging because they are set on concrete (but for some strange reason not when put on cold steel), problems from switching brands of engine oil, and even problems from mixing different brands of DOT3 brake fluid, are some popular examples.

Perhaps once in the distant past there was some truth to these stories, but certainly not today. My feeling is that many of these stories were made up by manufacturers to keep their flock from straying. Kind of like the stories that the 3-in-1 oil folks made up about WD40 containing water. That rumor is still being passed around... As is the long disproved battery/concrete floor myth.

I'm not going to say that over a long enough time frame, under just the right conditions, that you won't see some reaction between DOT3/4 and DOT 5; add in some water, rust, heat, and some ??? and something might happen. I am going to say that clean DOT3/4 and clean DOT5 will sit together without mixing for longer than I am willing to wait.

Ask me again sometime, and I will take some more pictures of my jar of DOT5 and DOT3.... I have a friend who has had his test jar for many many years now, and as far as I know, it is still unchanged.

OBTW, the DOT3 in my jar is from a bottle that had a leak in its bottom, and was resting on its side for about 25 years. It probably has plenty of water vapor absorbed into solution by now.

-Chuck

 

[808pants]

no shortcuts to getting system cleaned out

It was time for me to go through my brakes and either stick with DOT5 or change to DOT3. Like some others, I tend not to subscribe to the the thinking that mixing of DOT3 & DOT4 will do anything more dramatic than create two layers of immiscible fluids, but I was, on the other hand, concerned about wear and potential leaks at my wheel cylinders because one of them was clearly leaky. I'd seen this about the time I had been having obvious problems with my air-pack anyway, so it was time to go through things to see what needed repair.
1) I used compressed air to thoroughly blow out fluid from all lines, via a single central point (I threaded an adapter into the tee near the air-pack) with the idea of letting it come through successive bleed-valves at all six wheels, recovering maybe 12oz of silicone fluid in that process.
2) I followed with a gallon of denatured alcohol, which I alternately pumped through from a pressure-sprayer until it sprayed out the bleed-valves/blew through with compressed air.
3) Only then, with lines and hopefully wheel cylinders all clean, did I start removing wheel cylinders to look at them.
All cylinders still contained significant amounts of silicone fluid, as well as the alcohol - despite copious amounts of compressed air having been used.

This didn't really surprise me much, since the position of the brake-line connection at each cylinder is well above the centerline, and of course the bleed-valve, from which we might like to think everything would exit cooperatively, is well above that...so no amount of blowing or flushing is likely to replace or remove all fluid in the wheel-cylinders. Rather, each will just empty to the point where the remaining fluid swirls about in the remaining space, never completely exiting.

I'd not have cared much about this since I'd figured on having to inspect each wheel cylinder anyway...but thought it noteworthy that this left a lot of alcohol/silicone sitting in the wheel-cylinders, and for another, I found that 3/4 of the cylinders had ugly buildups of rusty, gritty particles, with attendant scoring of the pistons. Nope, that stuff hadn't magically blown out, either. Yes, that was after many years of 'nothing but DOT5.' I haven't gotten the last two out yet, but if they're like the rearmost two, the pistons will have razor-sharp edges and visible scoring, which I conclude comes from being honed against their bores by this fine rusty grit during long use.

True, most of my cylinders were functional and not leaking. But because of low-point corrosion from accumulated water - whatever the source might be, it's clearly there in most cylinders, or I wouldn't be seeing the gritty-rusty stuff floating around in them - I'd rather use DOT3 and plan to regularly pump new fluid through the system with the uber-simple/easy one-man pump-sprayer process, every couple of years. It's easier than an oil-change with that method, and faster, too.

--Dave

 

[stumps]

If you check the TM's, you will find that the recommended way of doing the switch over from DOT3 to DOT5, which was done in the late 1980's, was to use the pressure bleeder filled with DOT5 to flush the system of all of its DOT3... Very much like you did.

However, like you, I too have done that experiment, and found that the DOT3 never leaves the system. The DOT 3 is heavier than DOT5, and the bleeder valves are at the top of the wheel cylinders... they are made to remove air.... not to facilitate a DOT3 to DOT5 conversion.

Since these trucks were originally shipped with DOT3 in them, and were subsequently changed over to DOT5 using the technique specified in the TM, it is logical to assume that they spent most of their lives with a puddle of DOT3 in each wheel cylinder.

On DOT3: Brake systems are supposed to be totally sealed systems, but they aren't really. There is a pair of rubber pistons in each wheel cylinder that has brake fluid on one side, and atmospheric air on the other. These trucks were routinely run through deepish water, mud, you name it. The brake drums certainly filled up with water and mud when this happened. The mud and water certainly got into the rubber boots in the wheel cylinders too, rusting the air side of the wheel cylinders.... which is where the metal pistons you saw all rusted, scratched up, and honed to a sharp edge live...

When the brakes are in a relaxed state, the pistons are retracted into the cylinders by the heavy springs on the brake shoes. These trucks sat, and sat, and sat, for years in this relaxed state. Then they got used, where brake pedal action causes the pistons to travel into, and out of, the rusted area of the wheel cylinders (that is usually only exposed to air). When they again retract under spring pressure, they drag some of that rust, and moisture, into the brake fluid side of the piston. This is how DOT3 systems get contaminated with water, and end up with rusted wheel cylinders.

Wash, rinse, repeat...

The only time I have ever had problems with DOT3 in brake systems is in vehicles that sit idle for long periods of time. In every one, when I finally come back to it, I find the brake cylinders rusted up.

I have several vehicles that I have converted over to DOT5... correctly... and they never have problems. It is like their brake systems are frozen in time.

What you are describing is typical of what I always find in DOT3 systems that have been left idle for years.
Your system most probably had DOT3 left in the wheel cylinders from the DOT5 changeover that happened in the 1980's.

And lastly, if you look at any of the commercially made pressure bleeders that is made for DOT3, you will notice that it has a bladder that separates the DOT3 from the pressurized air. This is because the DOT3 instantly absorbs all of the water from any air that touches it. Pressurized air has the water vapor of normal air, only concentrated by the pressure. Unless you live in a desert, you run a strong risk of contaminating your DOT3 using the famous SS garden sprayer pressure bleeder.

-Chuck

 

[Neophyte]

Absolutely fantastic thread on brake fluid in our trucks! I mistakenly added Dot 5 to my M817 (which has Dot 3/4 in it....found this out after the fact when filling an EMPTY master cylinder). Not so worried about the little I added, but will convert to DOT 5 after reading TB 43-0002-87.