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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Retired Moderator

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.

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