1. 1979 Mercedes 240d automatic, 155,000 miles, including 35,000 miles by this owner. After 4 years of driving, the brakes went mushy, and the pedal starting noticeably drifting down.

2. Bled the brakes. Bleeding the brakes didn’t help. See my separate post for a picture of my coffee can brake bleeder.

3. Replaced the master cylinder and re-bled the brakes. No real improvement. Still a mushy pedal with drift to near the floor. Two hours to R&R and bleed the brakes.

4. Plugged the vacuum line to the booster. The pedal dropped almost to the floor without mushiness, but was solid. It took a lot of force to actually apply the brakes.

5. Reinstalled the vacuum line to the booster, and installed a vacuum gauge on the T just upstream of the brake booster. Vacuum reached about 18 or 19 in Hg. When the brakes were applied repeatedly, the vacuum dropped to almost zero but there the pedal pumped up to a decent height. Then, as the vacuum built back up, the pedal drifted down to where it was with no booster. (Near the floor.) There seemed to be a direct correlation between the rate of vacuum build up and the rate of pedal drift. Two hours for diagnosis.

6. Replaced the booster. To get in position, I pulled the driver’s seat and rested my hips on a stool just outside the driver’s door. Stack some phone books or 2 x 4's on the floor to support your shoulders. (I'm past the stage where I'm willing to just contort to get things done.) Three nuts are easy, sort of. I pulled the instrument cluster to try to get at the 4th nut, but that was just a waste of time. The upper left nut is accessed by running an extension (about 15") almost horizontal directly at the nut. The extension weaves through the harness, etc. You can’t see the nut, but you can “feel” a socket into position. This is all done under the dash. Each nut had a wavy washer underneath, making me think this was possibly the original ATE booster. The push rod clevis is connected to the pedal arm by an 8 mm bolt & nut, passing through a shouldered bushing. The new booster needed to have one side of the pushrod clevis drilled out to allow re-use of the original bushing, which has a 5/8 o.d. shoulder that seats in the booster clevis. (The aftermarket booster had a clevis with 8 mm/5/16 holes on both sides ... nowhere to mount the bushing. Wasting time & $20 on a 5/8 drill bit seemed like a better option than leaving the project hanging on a Saturday and going chasing parts again.) When reassembling, it takes a long extension magnet to position the wavy washer and nut on the upper left stud so you can start them with your fingers.

The old booster did have a modest amount of brake fluid inside. Maybe 1/4 of a cup. I don’t see any practical way to actually extract that amount of brake fluid from a booster in its installed position.

7. Re-bled the brakes. The pedal is soft when applied, but does lock up at about 2/3 of the pedal stroke. No more pedal drift. Eight hours to remove the booster, go buy a 5/8 bit, modify the clevis, reassemble and re-bleed the system.

8. Rebuilding the vacuum pump is next.