Wetness on brake fluid reservoir cap

[shail]

Whilst under the bonnet replacing my bonnet cable I noticed that the TOP of the brake fluid reservoir cap was a little wet.  Nothing major, but wet in top of the cap. I can only assume it's brake fluid, so want to sort it out asap.

Are there any replaceable o-ring in the cap?  I'm not sure how the cap is out together - but it appears to be a 2 piece cap - one bit which has the thread and the other bit fitted through the top of the cap for the sensor.

Any pointers? 

[Octoparrot]

I'd find out if it is brake fluid before assuming.

[LC5F]

The cap should not be an air tight seal

Any recent brake work done?
could just be pads replaced and the fluid got pushed through breather.

give it a clean, brake cleaner is best and just monitor

[shail]

I've sprayed it with cleaner now.  I'll check it in the week to see if it comes back. It was very minor, so may take a while to reappear. 

Not had any brake work done recently. 

[mjmallia]

The cap should not be an air tight seal

It should be an air tight seal the cap, as brake fluid is hygroscopic.

[pudding]

Maybe it was a bit full to begin with and this heat has expanded the fluid a bit?

[LC5F]

The cap should not be an air tight seal

It should be an air tight seal the cap, as brake fluid is hygroscopic.

No - I agree that most brake fluids are hygroscopic, but if it was an air tight system you wouldn't need to change you brake fluid every 2 years, this is No.1 location where the moisture gets into the fluid.
Plus the fluid needs free air to flow in and out of the master cylinder, any resistance to fluid flow could be disastrous in an emergency.