brake and clutch inspection windows - does this look right?

[AliSTair]

Hi all,

I have recently been thinking that I should bleed the brakes on my '94 ST1100...They perform well, but believe it is good practice to bleed them every couple of year just to keep them in peak condition.

I will confess I have never paid much attention to them before, other than practising emergency braking in the car park and feeling confident that they stop me and the bike well when pushed hard.  Anyway, I have attached a couple of images of the inspection windows of both clutch and brake reservoir, and was interested if others' bikes look like this?  I had expected to see a horizontal line showing the fluid level, but mine appear to have a circular pattern to the visible markings.  Im thinking that this isn’t right and that when I bleed the brakes I should attempt to clean the internal face of the inspection window using brake cleaner to get a clearer view of fluid levels

However the clutch has a  similar display which makes me think maybe they are meant to look like this?   Bleeding the clutch system isn’t something I familiar with so will have to do a bit of research before tackling this one, if required.  All advice gratefully received.

Al

[Biggles]

Looks alright.  The idea of the visiglass is to warn you if the level drops. The half-way mark is a minimum below which the system may start ingesting air when the bike is leaned over.

[Gadget]

They look OK, however....

The reason for regular fluid change is that the fluid is hygroscopic, which means it absorbs moisture from the air.

Over time due to heating and cooling, air gets into the top of the reservoir and the moisture is absorbed into the fluid.

The problem is exacerbated because the water is heavier than the fluid and sinks to the bottom of the reservoir where it tends to corrode the reservoir or get pumped into the lines.

The water is more compressible than the fluid which is what causes spongy brakes.

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[alans1100]

If you have bled brakes before the procedure is the same.

[Brock]

As said, the line is the min level.,

If there is no air gap iin the resevoir, then water cant get in...

The rubber liner actually moves down into the gap as the pads wear, to keep air away.

[Biggles]

The water is more compressible than the fluid which is what causes spongy brakes.

I'd wonder about the science of that.  The compressibility of water is very minimally more than that of oil.  One key issue is the corrosion you mention, but more importantly, if there is significant water in the slave cylinders when they get very hot, the water can boil, and steam is definitely very compressible giving spongy brakes.

At 40 MPa, there is only a 1.8% decrease in volume in water.

[Gadget]

The other issue with water versus brake fluid is boiling point.

This article addresses both compressability and boiling point.

It reflects what the Army motor mechanics taught me when we were serving Land rover and Unimog brakes.

https://en.wikipedia.org/wiki/Brake_fluid

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[AliSTair]

Thanks for the feedback.....still dont really understand how it looks like this and why it doesn't just show as a horizontal line like the oil level inspection window, but I'm no longer concerned that the levels are too low.

Al

[Brock]

dont really understand how it looks like this

Pretty simple really, the fluid level is well above the window, so you wont see a line.