Have you ever tried to squeeze a sponge between two fingers? If so, you will notice that there is a certain amount of resistance—but not very much. This is the type of sensation that you may feel through the ball of your foot when you try to decelerate a car that has faulty brakes. The pedal will not feel firm (as it should) and you will quickly realise that something is wrong. What could this be, and how should you react?
Air in the System
Typically, a spongy brake pedal tells you that there is too much air within the system. Actually, there shouldn't be any air present at all, as the setup relies on the use of non-compressible hydraulic fluid, which helps to multiply the force you apply to the brake pedal.
Sometimes, air can get in if a seal or a pipe is worn, or it can get in when you top up the brake fluid. While it will need to be removed, the vehicle manufacturer has nevertheless built in a process that will allow you to restore everything back to normal.
If you want to attempt this yourself, you will have to jack up the car and make sure that it is sitting on axle stands at each corner. Before you elevate the car, remember to loosen the wheel nuts first so it will be easier to remove them. Then, take off all the wheels.
How to Bleed Each Wheel
Begin at the front, driver's side corner and look for the bleed screw located on the back of the caliper, next to the brake disc. You will have to loosen this by using a ring spanner to open the valve connected to the hydraulic line. Before you do this, attach a clear plastic tube to the end of the bleed nipple and put the other end into a nearby container.
Look underneath the bonnet to make sure that there is a sufficient amount of brake fluid in the master cylinder before you begin, and if not, top it up. Put the lid of the cylinder to one side for the time being.
You will need somebody to help you, and they should sit in the driver's seat and apply pressure to the brake pedal gradually while you allow the fluid to be pushed through the pipe and into the container. As it does so, any trapped air will escape with it, and you will be able to see bubbles in the solution when you look into the container.
After your assistant has depressed the pedal several times, they should hold it towards the bottom of its travel while you retighten the bleed screw.
You will need to repeat this process on each wheel, making sure that there is enough fluid within the master cylinder before you begin every step.
Schedule a Service
Hopefully, this should have restored the correct amount of pressure, and the brake pedal should feel normal. However, you do need to schedule a trip to your mechanic for vehicle repair as soon as possible so they can find out how the air entered the system in the first place.