Spongy brakes are when the pedal has a mushy feel to it that seems to go away after pressing it multiple times at once but comes back after laying off the pedal. This is a big problem when getting the vehicle to stop. Spongy brakes are a result of air getting into the brake system, which can happen from leaks in the lines, too little fluid in the system or the replacement of a part like a caliper that opened up a brake line. More than anything, you need to purge the air from the brake system with what is known as “bleeding the brakes.”
Raise at least one end of the vehicle on jack stands and remove the wheels so that you can access the brakes.