Replacing brake pads is one of the most common repairs that DIY mechanics perform. For one, the procedure is relatively straightforward. Additionally, although the process is relatively straightforward, the savings in labor and parts are often substantial relative to a dealership or independent mechanic since brake pads are always replaced in sets. Combined with the fact that brake pads are wear items that require replacement numerous times throughout the lifespan of a vehicle, and it’s easy to see why brake replacement is so often attempted on a DIY basis.
One of the most common questions related to brake pad replacement that we get asked is whether or not it is necessary to bleed the brakes when you replace the pads. Now, it goes without saying that if your brake fluid has reached the end of its service life per the vehicle manufacturer’s recommendations, then of course you should break out your brake bleeding kit, replace your fluid, and bleed your brakes. The same is true if your brake pedal feels spongy, which would indicate that there is air in your system and it would benefit from a bleed. If you aren’t sure whether your brake fluid is still good, you can also use a relatively inexpensive tool called a brake fluid tester in order to assess the health of your brake fluid.
That said, if your brake fluid is still good and your brake pedal is nice and firm, the good news is that changing your brake pads doesn’t require you to bleed your brakes for most vehicles. The reason for this is that replacing your brake pads doesn’t require you to open the bleed screws on your calipers, so the procedure itself doesn’t allow an opportunity for air to get into your brake system.
That said, you still may need to open the cap and evacuate some fluid from your brake master cylinder once the new pads are installed, as changing your brake pads can raise the level of your brake fluid. The reason for this is that over time, you lose around 8mm of thickness on an average brake pad during its lifespan, depending on the manufacturer. To compensate for this loss of brake pad material, the piston or pistons in your calipers must extend further in order to keep your pads in close proximity (nearly touching) to your brake rotors in order to maintain brake responsiveness. When your caliper pistons are extended, there is more volume in your brake system that must be filled by your brake fluid.
Due to this, as your brake pads wear, your brake fluid level drops. If you’ve topped it off over time, when you use your caliper wind back tool to compress the pistons before installing new pads, you displace all the fluid that was displacing the pistons and shove it back into your master cylinder reservoir. This can push the brake fluid level well past the full mark, which then requires you to remove a little bit so your system isn’t overfilled.
In conclusion, when replacing your brake pads it’s usually not necessary to bleed your brakes, unless there is another compelling reason to do so while you’re already working in the area. Nevertheless, you still have to be aware of your fluid level after you swap in new pads, and may have to remove some if the level is too high.