Dot3,4,5, why

[lindyp38]

great questions...and comments.......

ive always used dot 5.........just dont mix.......with other dot fluids.......if ur not sure...flush...flush...flush......

 

[Heavysteven]

DOT 5, I won't use any thing else. I don't go cheap when maintaining brakes.

 

[storeman]

"doghead: So, after 20 minutes of searching several terms or combination of terms, I found the thread I was looking for. Please try the search function, along the way(just now) I found a bunch of good threads to read. "

Thanks Doghead.
I would have tried that (as i have for many other things) but i kind of figured that since this was a thread on brake fluid, this would be a logical place to ask the question and save the time and frustration, particularly when those with knowledge were already commenting.
jerry

 

[doghead]

I really just thought finding that thread(or any other one that says the same stuff) was easier than typing it all out again! There was no frustration involved!

I know I posted a thread on testing brake fluid, as well as a few others.

 

[littlebob]

DOT 3 or 4 are good for daily drivers, like you take to work. DOT 5 is preferred in a vehicle that is that is not used enough
to boil the moisture out on a regular basis. I didn't follow DH's link, but sure it goes in to more detail.

 

[m16ty]

Personaly I find the price difference a small price to pay as the dot 3&4 are hydoscopic and can destroy a brake system even when it just sits there. The dot 5 being silicone repels water and cotes the internals helping to prevent rust. "I'd rather dump oil on my truck than salt"

There's another school of thought that says that water will collect in the system with DOT 5. Because it won't mix the water collects in the low spots, still causing rust issues.

I honestly don't know which school of thought is right. I'm running DOT 3 in my deuce. Mainly because of the price and availability. I also mixed DOT 3 with DOT 5 one time by accident without ill effects.

 

[nk14zp]

There's another school of thought that says that water will collect in the system with DOT 5. Because it won't mix the water collects in the low spots, still causing rust issues.

I honestly don't know which school of thought is right. I'm running DOT 3 in my deuce. Mainly because of the price and availability. I also mixed DOT 3 with DOT 5 one time by accident without ill effects.

I also use DOT 3.

 

[SEAFIRE]

Locally this has been discussed a lot among us VFD's.

Two of our sister sister VFD's had M35A2 brake system failures using DOT 3/4. I do not know if the brake systems were flushed properly or not before changing brake fluids. The brake systems were basically toast and had to have major work done to them to get the trucks back in service. One of the trucks never was fixed right and ended up being sent back to the Forest Service for another.

It was enough to scare us into staying with DOT 5.

 

[storeman]

Doghead,
Thanks for your help.
Jerry

 

[cbvet]

I've rescued & restored a lot of cars through the years. The last few years I've been working on military vehicles.
If a vehicle sits unused for a long time, you're likely to have rust & corrosion in the wheel cylinders no matter what kind of fluid you use.
I see no real advantage in DOT 5.

 

[stumps]

DOT5 will not rust a wheel cylinder on the liquid side. The rust you might find will be on the dry side, and is caused by water that leaks past the boots into the dry side when you go fording, and mudding, and don't immediately tear the system apart and clean it like the TM's say you should.

DOT5 will not make a brake pedal spongy. What makes a brake pedal spongy with DOT5 is if you get too vigorous in pouring, and pumping the pedal on a dry master cylinder, and let the DOT5 mix with air. It can take a long time to get all of the tiny air bubbles out once you let them in.

In my experience, if you use DOT5, you will not have to work on the wet side of your system ever again... well, not unless you submerge the axles in deep water.

-Chuck

 

[ODdave]

I've rescued & restored a lot of cars through the years. The last few years I've been working on military vehicles.
If a vehicle sits unused for a long time, you're likely to have rust & corrosion in the wheel cylinders no matter what kind of fluid you use.
I see no real advantage in DOT 5.

I have never see rust in a dot 5 system. Im curious what kind of vehical had 5 & rust?

 

[m16ty]

DOT5 will not rust a wheel cylinder on the liquid side. The rust you might find will be on the dry side, and is caused by water that leaks past the boots into the dry side when you go fording, and mudding, and don't immediately tear the system apart and clean it like the TM's say you should.

DOT5 will not make a brake pedal spongy. What makes a brake pedal spongy with DOT5 is if you get too vigorous in pouring, and pumping the pedal on a dry master cylinder, and let the DOT5 mix with air. It can take a long time to get all of the tiny air bubbles out once you let them in.

In my experience, if you use DOT5, you will not have to work on the wet side of your system ever again... well, not unless you submerge the axles in deep water.

-Chuck

Chuck, I'm not disagreeing with you. I just want to pick your brain a little.

It looks to me like there will still be moisture that can enter the M/C with DOT5 in it (just like it can with a M/C filled with DOT3). Just because water won't mix with DOT5 doesn't mean water can't get into the system. To me, it stands to reason that if moisture gets into a DOT5 system that it will settle in the low points of the system. I could be wrong but I've heard this arguement before and it seems resonable.

 

[m16ty]

I have never see rust in a dot 5 system. Im curious what kind of vehical had 5 & rust?

I've seen rust (and lots of it) in trucks from GL. One would assume that all the trucks comming from GL right now were running DOT5 ( I think the trucks that had rust issues had "use DOT5" written on the dash). Now I will admit that there's no telling what was actually put into the M/C of a GL truck so that could have been the problem but I have seen rust in a supposedly DOT5 system.

 

[cbvet]

I have had a 1970 & a 1973 deuce. Both had rust & sludge in some of the wheel cylinders. Both had DOT 5. Of course I can't know whether they've ALWAYS had DOT 5, but I think they did when I got them.
My buddy's M35 with DOT 3 had similar problems.
I've rarely seen rust pits so bad they couldn't be honed out.
Again, I repaired a LOT of cars made from the '40s through the '70s. I never saw a problem that I thought was caused by or related to the use of DOT3.

 

[ODdave]

I guess its just an extra precaution. Dot 5 will NOT attract moisture like 3&4 will so when temp cycles cause the cast iron peices of the sytem to sweat it SHOULDNT draw it in. Who knows what gets put into the system before we get em. Could it be the boil temp is higher with dot 5 so they thought it was a better move?

 

[stumps]

I'll try to answer the three main questions here:

It looks to me like there will still be moisture that can enter the M/C with DOT5 in it (just like it can with a M/C filled with DOT3). Just because water won't mix with DOT5 doesn't mean water can't get into the system. To me, it stands to reason that if moisture gets into a DOT5 system that it will settle in the low points of the system. I could be wrong but I've heard this argument before and it seems reasonable.

Yes, water will drop to a low point in a DOT5 system. But that point is generally in the sump of the M/C. Any water that got past the sump, would probably never leave the master cylinder... but assume that it did, and made its way to the wheel cylinders while the system was being bled, it would potentially cause rusting. Typically, it doesn't because there is little or no oxygen in the trapped water, and the cylinder walls of the cast iron wheel cylinders will have soaked up a protective coating of silicone quite thoroughly.

One has to think about how the water would get into the system in the first place...DOT3 is so hygroscopic that it will draw moisture out of the air, and contaminate the fluid in the M/C sump. DOT5 cannot do this.

I've seen rust (and lots of it) in trucks from GL. One would assume that all the trucks comming from GL right now were running DOT5 ( I think the trucks that had rust issues had "use DOT5" written on the dash). Now I will admit that there's no telling what was actually put into the M/C of a GL truck so that could have been the problem but I have seen rust in a supposedly DOT5 system.

The switch over from DOT3 to DOT5 came in phases. First, the supply depots were sent DOT5, and ordered to return all of the DOT3 in their possession. And then the PMCS details were ordered to flush and replace the DOT3 as time became available, or when the trucks came in for brake service. The order said to use DOT5 to top off any system regardless of whether it contained DOT3, or DOT5. The powers that be felt that doing a power flush with DOT5 would do an adequate job of clearing a system of its old DOT3 fluids.

I have personally tried this, and I can tell you that it doesn't do a thing to the fluid in the wheel cylinders. I have taken apart wheel cylinders where I tried to power flush the DOT3 out with DOT5, and have found them to still be 2/3 full of DOT3!

People that ford a lot, and let the water submerge the brake drums completely will see failures even with DOT5. This is because water, and mud, always leaks past the wheel cylinder's boots, and gets into the space between the aluminum piston, and the cylinder walls... a space that is supposed to only contain air.

I guess its just an extra precaution. Dot 5 will NOT attract moisture like 3&4 will so when temp cycles cause the cast iron peices of the sytem to sweat it SHOULDNT draw it in. Who knows what gets put into the system before we get em. Could it be the boil temp is higher with dot 5 so they thought it was a better move?

The reason behind the switch was most certainly due to rampant brake failures when trucks with DOT3 fluid were left to sit for months at a time without going anywhere. Vehicles that sit unused for long periods are the only vehicles I have ever had a failure with DOT3.

I have had two sorts of failures with DOT5: One is due to leakage at fittings. DOT5 will creep through poorly done fittings that would have easily held DOT3. The other was due to leaking around a piston cup on a cylinder that had the wrong type of expansion spring.

-Chuck

 

[jaxsof]

Just a small historical perspective. The switch to air brakes was partially prompted by heavy trucks trying to stop, and BOILING their brake fluid. Now, when a fluid, lets say DOT 3 absorbs water, and the brakes get hot enough to boil said water, well, that moisture does not return to the MC for release to the atmosphere (And, as we have already established, DOT3 really likes water!) . It becomes a little bubble of an easily compressed vapor, and your pedal, if you are of the lucky sort, just feels mushy. If your luck runs like mine, it hits the floor, and you potentially end up using a tree to stop.

My personal feelings on the matter, DO Not Compromise on your brakes!! I dont care how good your truck,or car, runs, it ain't worth a tinkers darn if it won't stop! The correct fluid can be had, and if you excercise a little effort, can be had affordably. If you're loosing fluid, that is a problem you really need to address, before you meet the afforementioned tree.

Just my $0.02

Thank you for your indulgence.
John

 

[stumps]

Yep! Using DOT3 because you think it will adsorb any stray water and keep the system from rusting is, as they say, a fools paradise. DOT3 has a very high boiling point, and DOT5's is even higher.... but as soon as the DOT3 absorbs any water, its boiling point starts heading down towards 212F.

-Chuck

 

[JasonS]

Yep! Using DOT3 because you think it will adsorb any stray water and keep the system from rusting is, as they say, a fools paradise. DOT3 has a very high boiling point, and DOT5's is even higher.... but as soon as the DOT3 absorbs any water, its boiling point starts heading down towards 212F.

-Chuck

You HAVE to flush out DOT3 periodically unless your mc is sealed.

Brake fluid Q/A towards the bottom of the page.
Understanding Tire Size Ratings - Brake Fluid Comparison - Hot Rod Magazine