Check the fluid level on the master brake cylinder (found on the firewall on driver's side).... If fluid is low, you have a leak in the system. Add brake fluid and have a mechanic examine the brake system for a leak and repair. If the brake pedal rides low but is firm and the brake fluid level is normal....the low brake pedal could be caused by the rear drum brakes being out of adjustment (if the car is equiped with rear drum brakes). In any event, don't take a chance on malfunctioning brakes and get the system repaired ASAP.
John Carroll