break light comes on dash when i press the brake pedel?

hi i have a 1997 mazda panel van e2200 diesel i've noticed today that the break pedel is very hard to press & the brakes aren't working very well if the engine is just on idel if i rev it the pedel & brakes seem to work ok i've also noticed tonite that if i really press hard the break dash light comes on, i've checked the brake fuild & it appears to be at the correct level could this be a servo prob, any help gratefully recieved.
Keith