Sound as if you have a brake fluid leak,
Has the brake fluid reservoir got any brake fluid in it?
Have a look around the car behind each wheel for signs of fluid leakage.
Also look under the bonnet around the brake master cylinder.
Where the leak is is where the problem is, if at the back of the wheels then a slave cylinder or calliper piston seal has failed and will need to be replaced.
You dont say what make or model the car is which would be helpful.
Other ideas depends upon braking system
Leaking or faulty brake servo unit vacuum leak
Faulty brake accumulator
Maurice, February 2011
