Dot3/4 vs 5.1 for daily driver

[Rust_Belt_Pete]

I am curious which is a better fluid at keeping the rust away for a daily driver. Would it be the dot3/4 that absorbs the water or is it the 5.1 which doesnt interact with water and I am assuming letting the moisture stay inside the system and corrode it?
Does enough moisture even get in to make a difference?
Not talking about performance but just rusting issues from the inside.

 

[slacktide_bitog]

The most important thing is changing it regularly. That's more important than which one you use. EVen cheap Walmart DOT3 will be fine if replaced every few years.

There are some that claim to last longer, such as Bosch ESI6

 

[Rust_Belt_Pete]

But what about 5.1? Hoe does it get the moisture out of the system? I am assuming the water is heavier and settle son the bottom?

 

[Jetronic]

I am curious which is a better fluid at keeping the rust away for a daily driver. Would it be the dot3/4 that absorbs the water or is it the 5.1 which doesnt interact with water and I am assuming letting the moisture stay inside the system and corrode it?
Does enough moisture even get in to make a difference?
Not talking about performance but just rusting issues from the inside.

All 3 you mentioned absorb the water. You will have brake failure before rust is an issue

 

[Rust_Belt_Pete]

All 3 you mentioned absorb the water. You will have brake failure before rust is an issue

So essentially it is an irrelevant concern as far as rust is considered?

 

[Jetronic]

So essentially it is an irrelevant concern as far as rust is considered?

the higher the dot number the more chances of acid formation.

but the viscosity goes down which makes the brake system more responsive, especially esp and abs

 

[JHZR2]

DOT 4 has a higher BP but it has been said that it absorbs water at a faster rate.

Not sure about 5.1 in comparison. There is always some level of solubility.

The concern of rust is secondary to the concern of a dirty, moist fluid boiling in a high heat condition, and/or facilitating corrosion or scoring on pistons and parts. If the vehicle is in that wet of conditions, it’s likely that lines or other parts will corrode and fail on the exterior first.

 

[Rust_Belt_Pete]

the higher the dot number the more chances of acid formation.

but the viscosity goes down which makes the brake system more responsive, especially esp and abs

Interesting, I never heard of the acid issue. Im assuming that is bad for ABS module and various seals?

 

[97prizm]

But what about 5.1? Hoe does it get the moisture out of the system? I am assuming the water is heavier and settle son the bottom?

You're thinking of silicone dot 5 (why they named it like this I'll never know). DOT 5.1 iz hygroscopic and does absorb some moisture. Not sure why but I have seen some brake fluid reservoirs state "DOT 3 or DOT 4 only" on the cap.

 

[Rust_Belt_Pete]

Ah! thats the one ! thank you.

 

[barryh]

I am curious which is a better fluid at keeping the rust away for a daily driver. Would it be the dot3/4 that absorbs the water or is it the 5.1 which doesnt interact with water and I am assuming letting the moisture stay inside the system and corrode it?
Does enough moisture even get in to make a difference?
Not talking about performance but just rusting issues from the inside.

Dot 3

Dot 3, 4 and 5.1 are glycol based and all absorb water. 5.1 absorbs water fastest, then 4 then 3.

Brake fluid contains a corrosion inhibiter so provided the fluid is changed before the inhibitor is depleted then water absorption shouldn't cause corrosion.

 

[shortyb]

Use these to find out just how corrosive your fluid is:

 

[brandon9715]

Not to hijack the thread, but on a related note, is it fine to just exchange the brake fluid in the reservoir? I know the best is to bleed all the lines, but without a power bleeder that is difficult.

 

[edyvw]

I am curious which is a better fluid at keeping the rust away for a daily driver. Would it be the dot3/4 that absorbs the water or is it the 5.1 which doesnt interact with water and I am assuming letting the moisture stay inside the system and corrode it?
Does enough moisture even get in to make a difference?
Not talking about performance but just rusting issues from the inside.

It is "interacting," or it is hygroscopic.
The fluid that would not is something like Castrol SRF, which is hydrophobic. Two problems with fluid like that:
1. It is expensive ($80 for 1ltr).
2. It must be changed first time in larger quantities to 100% flush out old fluid, otherwise old fluid might create wet pocket which later turns into air pocket.

 

[bdcardinal]

You're thinking of silicone dot 5 (why they named it like this I'll never know). DOT 5.1 iz hygroscopic and does absorb some moisture. Not sure why but I have seen some brake fluid reservoirs state "DOT 3 or DOT 4 only" on the cap.

People don't know that 5.1 exists so they would see DOT 5 and put that in.

 

[barryh]

Not to hijack the thread, but on a related note, is it fine to just exchange the brake fluid in the reservoir? I know the best is to bleed all the lines, but without a power bleeder that is difficult.

I wouldn't describe it as fine but it's perhaps marginally better than doing nothing.

My take on it is that because of the hydroscopic nature of brake fluid when you exchange the fluid in the reservoir then the water content throughout the system will eventually average out at a lower level than it was. What I don't see is a mechanism for the corrosion inhibiter in the new fluid to reach other parts of the system.

There is a method of brake fluid replacement that doesn't require a power bleeder or any special technique. Gravity bleeding involves simply opening the bleed nipple and allowing the fluid to flow out under gravity. It's worked for me many times but it's a slow process especially on the rear brakes. Provided the reservoir is kept topped up there is no risk of introducing air. Using a modern low viscosity fluid helps a little as does warmer weather.

 

[dlundblad]

I was told 5.1 isn’t compatible with older seals. Is this correct? This is too bad because I believe it’s easier on paint than the 3 and 4.

 

[97prizm]

I'm bummed that the DOT banned the sale of ATE super blue racing fluid. Scca drivers would alternate between that and the gold fluid.

 

[KrisZ]

Lots of people use DoT 5 brake fluid in their vintage motorcycles and cars because it will not damage the painted surfaces. Other than that there is no real advantage to it. It still needs to be changed regularly and may even cause more problems since water can separate instead of being mixed in.

Converting to it can also be problematic if the system was filled with glycol based brake fluid before. It’s best to convert to dot5 when the brake system is fully rebuilt.

 

[Rust_Belt_Pete]

Lots of people use DoT 5 brake fluid in their vintage motorcycles and cars because it will not damage the painted surfaces. Other than that there is no real advantage to it. It still needs to be changed regularly and may even cause more problems since water can separate instead of being mixed in.

Converting to it can also be problematic if the system was filled with glycol based brake fluid before. It’s best to convert to dot5 when the brake system is fully rebuilt.

That was my main question. So the water does drop out and sit.