The brake pump relay on the firewall may be acting up. When it fails, you will have rock hard brakes until you get a new one.
However, you said the pump ran for 30 seconds, and then you had the hard brake peddle, and then 3 minutes later it was ok. Are you sure the pump didn't run for 3 minutes until it was back to normal? If it did, then more likely it is the accumulator.
Very simply...Start car, relay tells pump to run and fill accumulator up with brake fluid. Accumulator gets full (pressurized), and the pump shuts off. Now when you brake, the braking force comes from the energy stored in your brake accumulator, (the brake fluid under pressure). When the amount of energy stored in the accumulator drops to a certain point, the pressure switch on the pump sends a signal to the relay, and the relay kicks in again and tells the pump to pump more brake fluid into the accumulator to charge it back up.