A soft or spongy pedal is most likely the result of air in the brake system.  Air bubbles may have entered the system because the fluid level got too low.  The air could have been from a small leak in the brake hose(s) or a leak at the master cylinder.  Always check to make sure the cap is tight on the braking reservoir. Boiling your brake fluid will cause a soft pedal as well.  There could be a hole in one of your brake hoses.  There could be a tear in the brake caliper piston seal(s).  Incorrect size of the master cylinder (too small), or calipers not mounted square to the disc are other reasons.