Before you take any thing apart Bleed the entire Brake system (Front and Back). It sounds like there's air in the system. Check for leaking brake lines and calipers. If that doesn't work pull the front brakes apart to see if the piston moves when the pedal is pressed and depressed possibly piston freeze. If this all passes then check your porpoting vavle for leaks and air bubbles, Last is the brake booster/Master cylinder.