When applying the brakes, the pedal of my 2004 Holden Cruze goes soft and there is a spongy sound.
Any time you have a soft brake pedal, you also have the potential for sudden brake failure. On that basis, you really shouldn’t drive this car anywhere, just in case the entire system decides to fail at the worst possible moment.
Meantime, the noise you’re hearing could be the sound of a leaking brake booster which is losing vacuum and, therefore, braking pressure. But if that was the case, you’d expect the brake pedal to go hard and require more effort to push it, not the other way around.
In many cases, a soft brake pedal is an indication of air in the system. The system should be full of brake fluid, not a combination of fluid and air. That’s because brake fluid is incompressible and, therefore, transfers all the braking force to the brake pads and rotors, stopping the car effectively. Air, on the other hand, is very compressible, and its presence will take away the brakes’ efficiency leading to the soft pedal you’re feeling. But it’s not just a soft pedal; you’ll also experience longer stopping distances and less outright braking power when you need it in an emergency.
Your car may need a brake overhaul, new brake fluid or perhaps just a process called bleeding (where the air is removed from the system). Either way, don’t drive it any farther.