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Attempting to decode secret message....
Brakes, front or back, require inspection, detection, and, if needed, resection.
Always safety first. Work on flat, stable area. Tranny in P and parking brake engaged. Firmly block wheel diagonally opposite the one under investigation. Lift, place vehicle on stable jack stand on solid flat foundation. A hydraulic jack alone is NOT safe. Use a stand. Do not clean the caliper area with compressed air, blowing all that crap up and maybe into your lungs. I use a nose/mouth rubber seal respirator and safety glasses.
Read the following and ID the tools needed:
1. Above safety steps.
2. Check fluid level in brake reservoir. If over MAX line, turkey baster some out and discard properly. Pressing back the pads will push some fluid back up.
3. Loosen lug bolts. On mine I use a 24" 1/2" dirve breaker bar and a 17mm impact socket.
4. Raise vehicle on jack stand.
5. Remove wheel.
6. With a flashlight, inspect hard brake line, brake hose, caliper, general area for wetness (leaking brake fluid or grease from some suspension component).
7. Using large slip-joint pliers (a.k.a. appliance or plumbing fixture pliers), gently attempt to pry the brake pad away from the rotor one millimeter (1/16"). Jaws go on the brake disk metal backing tabs. If movement detected, good; no movent = maybe a frozen piston and the entire caliper will have to be replaced (best in matched pairs, especially if you have a Bendix brand).
8. Style "A". Using needle nose pliers, remove the tiny retaining pins from the ends of the pad guide pins, and/or...
8. Style "B". Using a 3mm diameter long drift, gently punch out (small ball peen hammer) the guide pins, one at a time. Noting how they rest, then remove the pad retaining springs.
9. Depending on the vehicle, you may have pad sensor wires in front only (W123), and in the rear (W124 wagon). This means you need to have new pads and four or eight of these sensors on hand.
10. Get replacement pads only from reputable MB parts websites. The only time I got pads from Advanced Auto, the holes in the pads that were to receive the above sensors were either missing or not aligned with the notch in the metal pad backing that holds the sensor in place.
11. With the pads out, look around for rips, tears, gouges, etc.
12. Check rotor for similar uneven wear, warping, wobbles, then check thickness with a caliper. Ensure you are over the “rim” that is usually rusty and remaining at the outer edge of the rotor. If on the limit or below, replace with new rotors (a.k.a. brake disks). If you are going to do the rotors, do the bearings. See other threads on how-to DIY.
13. With the wheel out, check past the caliper at all suspension and steering components. Grab, tug, twist, pull everything back there listening for teltale clicks whle looking for old, cracked rubber, anything with grease on it, and broken linkage connection boots. With parts in hand, the linkage damper on mine can be popped out using a couple 17mm wrenches, a ball peen hammer, and a pickle fork. Torque wrench the nuts back to spec.
14. Press back the pistons carefully with a special piston tool or the appliance fixture pliers. Take care not to bite anything fragile.
15. Check the reservoir again and remove fluid as may be required.
16. Reassembly is reverse with additional, as follows.
17. Depending on the manufacturer of the new pads, a small tube of pad lube anti-squeak may be included in the pad set. Follow directions carefully on how much on which pad per caliper. Clearly none goes on the wear surface.
18. With the wheel back on and still up off the pavement, give it a spin and listen for rhythmic scraping. This may indicate a warped rotor. Replace rotors in pairs. Then grab the wheel at 12 and 6 and push top, pull bottom, reverse. This is a simple check for a loose wheel bearing. Dittos 3 and 9, this for loose steering components.
19. Start the engine and press the brake pedal a few times to do an initial seating of the pads on the rotors.
20. Check and fill the brake fluid reservoir with new, clean fluid. Do not get any on the paint; do use the integral screen under the filler cap.
21. Take out for a test ride on a not too busy road. Residential areas are good since you want to stop many times from low speeds, then go out on some back roads to pick up the speed and apply the brakes. This is the proverbial “breaking in” the new pads. There might be some aroma of the glaze burning off with normal friction heat.
22. Pedal should be strong by now. If it is mushy or soft, you have air in the lines and will need to do a bleed. See threads.
23. If the fluid in the reservoir is orangey or hasn’t been changed in a year or 15,000 miles, say, you should do a flush. See threads.
Bottom line, the proper functioning of this system may mean life or death. Do each element carefully and completely. Use a checklist if needed. Inspect everything for indications of other issues such as line failure, uneven wear, leaks.
I think that's about it. Noble Forum Fellows hopefully will check and add anything I may have forgotten. You should Google other methods and opinions as well.
Good luck. Let us know how you do.
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