Had the same on mine and it got worse, turned out to be the clutch master cylinder (common problem on these) but will need fixing since it shares the brake fluid resevoir, keep an eye on the level until fixed. Bought new one from ford for just under £80, fitted it myself using haynes manual, took a couple of hours, difficult bit is reaching into engine compartment to access pipes, haynes says to jack up car to do this from below, but I found it easier to reach in from above. Also to reach the clutch bleed screw you need to remove the air filter & a few related pipes, not too difficult though. Good Luck
Trev, January 2008