Nekkid

Hi folks.
The bike went into the garage for a service last week and I asked them to look at the brakes cos they're feeling really rubbish at the moment. He said he gave them a clean so the cylinders were moving freely. Fair enough, I know they get a bit sticky. The thing is, the front brake reservoir is just (and I mean just) above the minimum line in the viewing window. I did ask if he filled it, but he said it didn't need it. Was he talking crap? Are the brakes less effective when the level is low?
So, if I'm going to fill it myself (bl00dy mechanics!), do I just unscrew the top of the reservoir, fill it then screw it back on, or do I have to bleed the system or anything like that?
Cheers for any advice!