Brake fluid warning sensor

by FrontBackSide2Side Fri Dec 05, 2014 9:26 am

The brake fluid light on the dashboard is more or less constantly on in my truck. The problem started when i would be going over speed bumps etc the light would momentarily flash on, then it started getting worse and it would stay on for a few seconds. Now it has got to the point where it is on more than it is off.

I've checked the fluid and it is halfway between min and max, im reluctant to add any more as this will cause me problems when i change the pads later down the line.

I read on another thread that removing the cap on the brake fluid reservoir then replacing it can cure this sometimes so i've tried that to no avail.

When i did take the cap off the underside of it looked deformed and mishapen as if the air had been sucked out of it, is this normal? Also there was something that looked like a white doughnut floating in the fluid, is this normal?

Any other ideas?

by sputnik Fri Dec 05, 2014 10:22 am

Assuming the Hijet is the same as my motorcycle brake fluid reservoir, there is a soft rubber gasket/diaphragm that sits on the lip beneath the cap which is supposed to hang down from the lip of the reservoir to roughly the top level of the fluid. It is supposed to reduce the degrading effect of air on the fluid (effectively just by taking up the available space instead of having a big air gap).

In fact, I've just looked at the service manual and the Hijet is the same.

I suspect your doughnut is this diaphragm that has squelched out of shape and fallen into the fluid? You should be able to fish it out, squeeze it back into shape and put it into place. It is unlikely to have done any harm. Just make sure you don't drip brake fluid on your dash/paintwork as it eats into such things pretty quickly. As far as I can recall, the only issue is making sure the diaphragm is not inside out (which might be why it fell into the fluid in the first place)

Alternatively, it may in fact be a doughnut. A quick taste test may confirm?

by sputnik Fri Dec 05, 2014 10:24 am

sputnik wrote:Assuming the Hijet is the same as my motorcycle brake fluid reservoir, there is a soft rubber gasket/diaphragm that sits on the lip beneath the cap which is supposed to hang down from the lip of the reservoir to roughly the top level of the fluid. It is supposed to reduce the degrading effect of air on the fluid (effectively just by taking up the available space instead of having a big air gap).

In fact, I've just looked at the service manual and the Hijet is the same.

I suspect your doughnut is this diaphragm that has squelched out of shape and fallen into the fluid? You should be able to fish it out, squeeze it back into shape and put it into place. It is unlikely to have done any harm. Just make sure you don't drip brake fluid on your dash/paintwork as it eats into such things pretty quickly. As far as I can recall, the only issue is making sure the diaphragm is not inside out (which might be why it fell into the fluid in the first place).

Probably worth putting a dribble more fluid in to get it back nearer to the max - I can't see that this would cause any problems when you change the pads as long as you don't go above the max.

Alternatively, it may in fact be a doughnut. A quick taste test may confirm?

by HighlyJetted Fri Dec 05, 2014 10:26 am

No, there is a donut shaped float that sits in the fluid.

In the middle there is a little magnet.

This interacts with the reed switch which detects low fluid.

Maybe the magnet has fallen out.

by sputnik Fri Dec 05, 2014 11:11 am

Quite right - I stand corrected! I gave a quick look to the service manual and it looked pretty much like my bike diaphragm but looking again I see that right next to it on the diagram there is also a reservoir float... which is much more doughnutty!

by bushwhacker Fri Dec 05, 2014 4:25 pm

I've had this problem many times. Usually it is cured by removing / replacing the cap but once the fluid gets halfway or less I found that it had to be topped up properly.

And yes, there is a float in the fluid.

by FrontBackSide2Side Fri Dec 05, 2014 8:11 pm

Ok cool, cheers guys, i'll check your suggestions over the weekend Smile

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