Replacing your brake pads is one of the easiest jobs to perform on your Mercedes-Benz. In general, you should inspect your brake pads about every 10,000 miles and replace them if the material lining of the pad is worn down enough. In reality, most people don't inspect their pads very often and usually wait until they loose braking power or start doing damage to the rotor. While the 190e models only have sensors on the front brakes, it's a wise idea to replace the rear pads and inspect your discs as soon as you see that warning lamp go on for the front brakes.
If you ignore the warning lamp, you may indeed get to the point of metal-on-metal contact, where the metal backing of the pads are contacting the brake discs. Using the brakes during this condition will not only give you inadequate braking, but will also begin to wear grooves in your brake discs. Once the discs are grooved, they are damaged, and there is often no way to repair them. Resurfacing will sometimes work, but often the groove that is cut will be deeper than is allowed by the rotors specifications. The smart thing to do is to replace your pads right away.
Brake pads should only be replaced in pairs--replace either both front pads or both rear pads at a time. The same rule applies to the brake discs that should be checked each time you replace your brake pads.
The procedure for replacing pads on all the wheels is basically the same. There are slight configuration differences between front, which have floating calipers and rear brakes, which have fixed calipers, but in general the procedure for replacement is similar. This article will show you how to replace your rear pads, if you are doing the fronts; refer to our Pelican Parts Technical Article on replacing your front pads. The first step is to jack up the car and remove the wheel (slightly loosen the lug nuts before you lift the car off of the ground). This will expose the brake caliper that presses the pads against the disc. Make sure that the parking brake is off when you start to work on the rotors.
The pads are held within the caliper by two retaining pins. There are also small retaining clips that hold these two retaining pins in the caliper. Start by tapping out the retaining pins using a small screwdriver or punch and a hammer. When the two retaining pins are removed, the anti rattle spring that holds the pads in place will fall out. Note; In high corrosion areas the caliper may need to be dismounted to hammer the pads out, then file the caliper body/pad side contact area to make the new pad installation possible. If there is excessive resistance from either or both caliper pistons while compressing them, this is a strong indicator they need to be rebuilt or replaced now. Now the pads can be pried out with a screwdriver. Work the screwdriver between the rotor and outboard brake pad. This allows you to pry and press the piston back into the caliper with no risk to the rubber dust boot. Be aware that as you push back the pistons in the calipers, you will cause the level of the brake reservoir to rise. Make sure that you don't have too much fluid in your reservoir. If the level is high, you may have to siphon out a bit from the reservoir to prevent it from overflowing. Use the small holes on the pads that normally surround the retaining pin as a leverage point for removing them. They may require some wiggling to remove, as it is sometimes a tight fit. It is important to keep in mind that the caliper piston is also probably pressing against the pads slightly and will add to the difficulty in removing them.
Once you have the pads removed, inspect the inside of the caliper. You should clean this area. Make sure that the dust boots and the clamping rings inside the caliper are not ripped or damaged. If they are, then the caliper may need to be rebuilt.
At this point, you should inspect the brake discs carefully. Using a micrometer, take a measurement of the disc thickness. If the disc is worn beyond its specifications, then it's time to replace it along with the one on the other side. Most discs have the minimum thickness stamped on them, if yours don't, please refer to your owner's manual.
The installation of the new brake pads is quite easy. You will need to take a small piece of wood or plastic and push the caliper piston back into the caliper. This is because the new pads are going to be quite a bit thicker than the old ones, and the piston is set in the old pad's position. Pry back the piston using the wood, being careful not to use too much force. Using a screwdriver here is not recommended as it can accidentally damage the dust boots and seals inside the caliper. Make sure that you push both pistons (inside and outside) back in the caliper.
Be aware that as you push back the pistons in the calipers, you will cause the level of the brake reservoir to rise. Make sure that you don't have too much fluid in your reservoir. If the level is high, you may have to siphon out a bit from the reservoir to prevent it from overflowing. Also make sure that you have the cap securely fastened to the top of reservoir. Failure to do this may result in brake fluid accidentally getting on your paint.
When the piston is pushed all the way back, you should then be able to insert the pad into the caliper. If you encounter resistance, double check to make sure that the inside of the caliper body is clean. When the pads are in place, insert the retaining pins and spring clip back into place. It's wise to use a new set of pins and clips when replacing your pads. Make sure that you replace the pin retaining clips inside the small holes in the retaining pins.
In general, I recommend removing and replacing the brake pads one side at a time. When the piston is pushed back into the caliper, it will try to push out the piston on the opposite side of the caliper. Leaving the brake pad installed on one side keeps the piston from being pushed out too far.
You also may want to spray the back of the brake pads with some anti-squeal paste. This paste basically keeps the pads and the pistons glued together and prevent noisy vibration. Some brands of pads may come with anti-squeal pads already attached to the rear surface. Anti-squeal pads can also be purchased separately as sheets that are peeled off and stuck on the rear of the pads.
When finished with both sides, press on the brake pedal repeatedly to make sure that the pads and the pistons seat properly. Take care not to push the brake pedal all the way to the floor, as you can actually damage the master cylinder by driving the piston into usually unused portion of the master cylinder and damaging the seals. Also make sure that you top off the master cylinder brake fluid reservoir if necessary. Brake pads typically take between 100 and 200 miles to completely break in. It's typical for braking performance to suffer slightly as the pads begin their wear-in period. Make sure that you avoid any heavy braking during this period.
To remove the old pads, tap out the retaining pins (green arrow) with a screwdriver and a small hammer. They should slide out pretty easily, as there is usually no load on them. If there is much difficulty encountered during the removal process, then tap on the pads slightly to remove pressure from the pins.
Pulling out the pads usually involves the use of a screwdriver for leverage. The pads are loose in the caliper, but it's a pretty tight fit, and there is usually lots of dust and debris in the caliper. Wiggle the pads back and forth in order to pry them free. Although these parts usually can be reused, some people prefer to install new retainer kits. The kits include two new retainer springs, and four pins that are used to hold the pads into the caliper.
When you are ready to install the pads back into the caliper, use a wooden or plastic handle to push back the two caliper pistons (yellow arrow). Don't use a screwdriver, as you might damage some of the piston seals. Keep your eye on the fluid level in the master cylinder reservoir--it can overflow when you push back on the pistons.
Install the new pads into the caliper. You may have to install pads into one side of the caliper (red arrow), and then push the piston on the other side back in to get the next pad in. Don't forget to reinstall the small retaining clips for the pad retaining pins. The completed assembly should be carefully tested before you do any performance driving. Brake pads can also take several hundred miles to fully break themselves in. Exercise care when driving with brand new brake pads.