laguna mkii losing brake fluid

Hi freinds, hopefully there's a easy solution to my problem, my laguna 1.9dci keeps loosing brake fluid from the resivoir cap. Every time I screw the cap back on and clean up the fluid off the bottle, I will go for a drive and again the cap will have come lose and there will be fluid coming out. I clean it up and screw the cap back to get the same problem every time I check after a journey. The fluid level is still ok at the moment but can anybody help me to sort this out. it worries me as its brakes and fluid and I have kids in my car most days. Any help will be appreciated, Thanks.