DOT 3 and 4 brake fluid is hygroscopic, which means it sucks moisture from the air, moisture+air+steel=RUST...I HATE RUST!!!
I am in the process of restoring a '68 Ford Ranchero. I was pissed when I had to toss out every single component that was in contact with the DOT 3 fluid, so I searched for a solution and found Dot 5.0. DOT 5.0 (not to be confused with DOT 5.x) is silicone based, and not hygroscopic.

The question is: can I get rid of the nasty DOT 4 in my W123, and use dot 5.0?

Richard

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'83 300 TD..."Helga" Norse for holy... a liittle rust problem
'85 300 TD...A $200 rear-ended total..being transformed into a pick-up truck
'68 Ford Ranchero GT...a pick-up truck that Ford transformed from their wagon

Only if you replace EVERY brake component first...

Any residual dot 3 or 4 will cause failure...

I'm sure that others will say otherwise but I have seen several cars totaled due to that fluid in-compatilibility!

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MERCEDES Benz Master Guild Technician (6 TIMES)
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Retired Moderator

i did my research on better brake fluid and decided that wagner 5.1 would be my replacement for 3-4.
i have been replacing the fluid thru a flush process and not replacing any components.have 2 years + and its been done on at least 20 vehicles and i havent experienced any problems at all.
hope this helps
larry perkins lou ky 87 old cars

Absolutely agree! DOT 5 silicone-based fluid is fine for a vintage car and will protect the system from corrosion, but you MUST remove every last molecule of the glycol ether-based fluid. This means new or rebuilt hydraulic components that were assembled with DOT 5 fluid. If you start with store bought new or rebuilt master or wheel cylinders, disassemble them, clean them with denatured alcohol and reassemble with DOT 5.

Buy new hoses and clean them as above.

If you don't buy new hard lines, thoroughly flush with alcohol and dry with compressed air.

When switching to DOT 5 the only way to go is start with everything clean and dry and begin assembly from scratch with DOT 5.

Unfortunately, domestic manufacturers never told us to change the brake fluid like Mercedes and most of the other European OEMs, and I learned 25 years ago that it was necessary for any car that you intend to keep a long time if you don't want to rebuild/replace the hydraulic components every 5-10 years.

The other solution is DOT 5, and if you're doing a frame up resto, that's a good time to convert, however, DOT 5 should NOT be used in ABS systems as it can cavitate under rapid ABS valve pulsation.

Duke

There are other contrasting opinions on DOT3/4 vs. DOT 5 fluids. DOT3/4 will absorb the water from your system as it is intended and will be flushed out with each required fluid change. DOT 5, since it does not absorb water, will still get moisture in it, but the water droplets will gravitate to a place somewehere in your brake system, set up house keeping and start the rust to start. I tend to agree with the DOT3/4 proponents and change the fluid as required by the book. I've got an '83 240D that I've had since new and a '59 220S that I've had for ten years and have never had rust problems caused by the DOT3/4 fluid that gets yearly flushes per the book.

Len

Good points. Vintage Corvettes in particular can "pump water" (and air) into the system due to rotor runout, and other disk brake designs may be similarly affected. Drum brake designs are somewhat immune from this because they aren't subjected to direct splash or piston oscillation when the brakes are retracted.

But vintage cars are often not, at least intentionally, driven in the rain and are usually garaged, so they see less water splash and condensing humidity than a typical daily driver.

DOT 5 fluid does not eliminate the brake fluid change requirement, but it means that on a typical well cared for vintage car, you can probably go 5-10 years without ill-affect, especially if it's a domestic that has a "sealed" master cylinder cover. Most European and Japanese designs have a vented fluid reservoirs, which is a moisture migration path.

Duke

I thought it was the opposite, that you need to change DOT-5 more often because it does not absorb water and thus lets it pool.

guys go read 5.1 is not synthetic.
larry perkins

I do believe that the discussion specifically excludes DOT 5.1.

Why would they make a 5.0 and a 5.1 so dramaticly different knowing Joe Blow would think they are the same....

Defies logic.


Personally I stick with what it came with originaly...and change it every year or two. THe stuff isn't that bad that these cars have ran 25+ years sometimes with what it left the factory with.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

the silicone fluid successfully in my vintage cars that have drum brakes. absolutely excellent! actually cures small seeps.

i tried it in my benzes and it seems to cause the pucks to stick and drag. so i reluctantly dont recommend in the benzes. unless old enough to have drum brakes.

so larry what is the deal with 5.1? what is it?

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

As luck would have it, Road & Track has a mention of this very subject in the Tech Tidbits column.

"DOT-3 brake fluids are glycol-based. DOT-4 fluids contain borate esters as well. DOT 5.1 fluids have advanced chemistry of a similar makeup."
[...]
"DOT-5 brake fluids are silicone-based and are not hygroscopic. Thus, they do not require periodic replacement."
[...]
"But silicone fluids are not without their tradeoffs. Some brake-system components of silicone rubber are attacked by these fluids. What's more, any moisture—and some is inevitable—tends to form globules that seek out a low point in the system. There, these water bubbles can degrade performance even worse than a glycol's absorbed moisture. Last,silicone brake fluids absorb air at a rate three times that of their glycol counterparts. This can lead to a spongy pedal and less than optimal ABS operation.
The two types of brake fluids, glycols and silicones, are utterly incompatible. In fact, DOT-3, -4 or -5.1 mixed with DOT-5 can result in a coagulated mess."

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1989 300 SEL that mostly works, but needs TLC

The key point is to change the fluid every two years regardless of what make of car. MBZ requires this, the Ford and GM are mum on this unfortunately.

I also own a 67 and 69 Corvette and am EXTREMELY familiar with the leak problems on this state of the art brake system for it's time. As a note to Duke, I neglected my 67 Coupe with stainless steel sleeved calipers. After eight years (2000k miles total) they started leaking. I rebuilt the same calipers with the new o-ring pistons. (no internal springs). SO far no problems, but I will change fluid more often.

NOTE*** all the heavy bow tie Chevy mags are 100% : FORD brake fluid is the best. Just the stuff from the dealer parts dept. The parts guys laugh at me but say I am not the only bow tie guy purchasing Ford brake fluid.

Racers like the Ford fluid because I think it has a higher dry boiling point than many DOT 3s, but it's only rated DOT 3 because the wet boiling point doesn't meet the DOT 4 wet spec. Racers often flush prior to every event.

My research over the years indicates (but it's not unequivocal) that DOT 4s offer better long term corrosion protection so that's what I use in all my cars except the SWC that has DOT 5.

I alternate between kicking myself and congratulatiing myself for not having disk brakes and FI, but, no doubt, in the long run, my (J-65 metallic) drum brakes and AFB have been a lot less hassle.

Duke