As xs 11 said, check brake fluid level. If one of the brake lines - usually one of the short hoses at the front wheel calipers - is weak, hard braking can be enough to cause it to split.
If the fluid level is low, top it up and press the pedal a couple of times, any leak will be fairly obvious.
If the fluid level is OK, you may have a failed seal in the master cylinder.
klf, January 2010