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.
Attach a clear rubber tube to the bleeder valve located on the brake caliper. Place the other end of the hose into a container partially filled with brake fluid.
Turn the bleeder screw on the caliper to open the bleeder valve, and have another person press down on the brake pedal from inside the vehicle. This is called "bleeding" the brakes, which purges air from the system. Continue applying the pedal repeatedly until only fluid cleanly comes out the tube.
Repeat the process with the other brakes. Replace the wheels after the brakes on one end have been bled, then lower that end and switch to the other end of the vehicle.
Top off the master cylinder with fresh brake fluid after all the brakes have been bled.
Check for leaks in the brake system, especially around the brake lines, hoses and fittings to the master cylinder or calipers. Replace dry, cracked hoses and worn seals or fittings, because they leak air into the system along with losing fluid. It helps to start the bleeding process at the brake caliper farthest from the master cylinder and working your way to the closest.
There can be leaks in the brake system in areas other than the lines or the fittings. These leaks can be very difficult to locate for someone who isn't an expert on braking systems.