1991 Honda Accord has spongy brakes when going at highway speeds. When I hold the brakes on in town it is fine but at faster speeds the brake peddle almost hits the floor but still slows down the car, if I push the brakes the rest of the way then the tires slide. The brake peddle will get firmer if I pump the brakes.

Any Idea What could cause this. Only thing I could think of is master cylinder.

 

^^ yup

 

master cylinder...
20 years of usage will take a toll on the seals.
change it...sounds very dangerous to drive on.