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My D9's brake fluid level drops slightly everyday after a bit of driving, but there are no visual leaks as far as I can see. What could be the problem?

Brake fluid will drop every time you drive it but it wouldn't be enough to really notice so I suspect this is a significant amount. How much? Enough to set off the low fluid warning light or just a few mm? Has any work been done on the braking system? What have you checked? You'll have to have a good look at the calipers (your car has disks all round?), brake hoses, brake lines. It's a saloon so no load valve on the rear subframe? The only other place I've lost brake fluid from is from the seals in the master cylinder, the fluid was going into the servo and from there into the engine (diesel or petrol?) but I'd put this idea somewhere between very unlikely and impossible.

Yep,if the car is loosing fluid constantly you need to pay attention to this,cuz its serious.
Look in the calipers bleed valve,like steve said,check all seals,check perhaps even the plastic box which contains break fluid for possible cracks.
How does it brakes,good?
Clean everything on master cylinder,and other brake components and maybe you cann see where it becomes oily again

Thanks guys. I don't lose as much to get the warning, but few mm like Steve said. Its a manual petrol D9, and its got discs in the front and drums in the rear. No work was done on the brake system since I got it, and the brakes are ok too. I will clean the brake components and see if it gets oily again.

CorrosiveKid wrote:drums in the rear

In that case a wheel cylinder is odds-on favourite.