Brake fluid spec question

[Herb]

This is up my street, unlike the dyno stuff which I took a while to get my head around. I used to work in brake NVH so have a reasonable understanding of this stuff.

DOT 3,4 and 5.1 are alcohol based and absorb water from the atmosphere. On older bikes moisture could even be drawn in through the brake lines, but some moisture will be drawn in past seals. Over the service life of the system, a couple of percent water can be drawn in.

The reason why this is a good thing on a road bike, where you don't want to be changing brake fluid all the time, is because although you get a drop in the boiling point, and a drop in performance over time the water is absorbed into the fluid.

On a system with DOT 5, any atmospheric moisture can pool in the system so can cause corrosion, or worse, brake fade when the water boils.

Where DOT5 is chosen, race bikes for example, due to the higher boiling point, it tends to be frequently re-bled so moisture is not an issue.

[Corvus]

This is up my street, unlike the dyno stuff which I took a while to get my head around. I used to work in brake NVH so have a reasonable understanding of this stuff.

DOT 3,4 and 5.1 are alcohol based and absorb water from the atmosphere. On older bikes moisture could even be drawn in through the brake lines, but some moisture will be drawn in past seals. Over the service life of the system, a couple of percent water can be drawn in.

The reason why this is a good thing on a road bike, where you don't want to be changing brake fluid all the time, is because although you get a drop in the boiling point, and a drop in performance over time the water is absorbed into the fluid.

On a system with DOT 5, any atmospheric moisture can pool in the system so can cause corrosion, or worse, brake fade when the water boils.

Where DOT5 is chosen, race bikes for example, due to the higher boiling point, it tends to be frequently re-bled so moisture is not an issue.

Thanks herb.

Can you explain how water gets in there with dot5? I thought silicon based stuff repelled water? Past the seals I guess, as you said for the other grades.

[Herb]

This is up my street, unlike the dyno stuff which I took a while to get my head around. I used to work in brake NVH so have a reasonable understanding of this stuff.

DOT 3,4 and 5.1 are alcohol based and absorb water from the atmosphere. On older bikes moisture could even be drawn in through the brake lines, but some moisture will be drawn in past seals. Over the service life of the system, a couple of percent water can be drawn in.

The reason why this is a good thing on a road bike, where you don't want to be changing brake fluid all the time, is because although you get a drop in the boiling point, and a drop in performance over time the water is absorbed into the fluid.

On a system with DOT 5, any atmospheric moisture can pool in the system so can cause corrosion, or worse, brake fade when the water boils.

Where DOT5 is chosen, race bikes for example, due to the higher boiling point, it tends to be frequently re-bled so moisture is not an issue.

Thanks herb.

Can you explain how water gets in there with dot5? I thought silicon based stuff repelled water? Past the seals I guess, as you said for the other grades.

Mostly, the moisture would already be there. It does not get drawn into the system the way it would with alcohol based fluids.