If the pedal softens soon after the engine has started and you retain full braking power with normal pedal effort then this suggest the fault is not critical, but the pedal being hard after the engine being switched off suggests there is a loss of vacuum pressure. This could be caused by a leak in the vacuum pipes from the brake servo or possibly a faulty check valve which is designed to retain vacuum pressure once the engine is switched off. We would suggest returning to the dealer and asking them to check the system, but if you are unhappy with their service you may wish to consult an independent Kia specialist.