Spongy brakes are generally the result of air, gas or water inside the hydraulic system. Car brakes use a hydraulic fluid that transfers force through high-pressure brake lines. If you’ve got something other than brake fluid inside those lines, the brake fluid is less efficient in transferring force. That leads to a spongy feel at the brake pedal.
The most common causes of this trouble stem from the brake fluid itself. If there’s any water in the brake line, heat from the brakes can vaporize the water, resulting in that spongy feeling. Or in some cases, the brake fluid can actually boil if the brakes get too hot.
The easiest thing to do about spongy brakes is to change the fluid. In most cases, that will get the brakes back to operating normally, unless there’s another problem with the brakes. In general, it is wise to have your brake fluid changed periodically. Besides causing spongy brakes, water in the brake fluid causes corrosion that can ruin calipers and brake cylinders. Brake fluid changes are relatively cheap, so you should have it done every year or two. That’s all part of proper maintenance for your car, as covered in Top 10 Car Care Tips.