Stiff brake pedal

[Delorean17@xxxx]

Hello List,
When I drive my car the brake pedal does not go down as far as it it 
should after about 5 minutes of driving. (moves only about an inch to come to 
a complete stop.) Then at the next stop it will be fine again and move like 
it should, then the problem might come back a little bit later on the drive. 
Any ideas on what this could be? The brake fluid is full and looks fairly 
clean. Is it safe to drive the car this way? 
also, is it legal to drive a car without a front fender?

Thanks in advance!
David