I had something interesting happen to me this morning and I can't quite explain it. I had a 20 minute or so highway drive returning from court in an adjacent county. I got into town and drove all the way through with no problems. As I neared my destination, I lost a significant portion of my brake pressure and as I sat at a light I could feel the pedal going down to the floor.

I reached my destination, went in, came back out 15 minutes later, and checked out the car. The master cylinder was full, there was no evidence of leakage at any of the calipers or lines and all the vacuum lines were connected. I started the car and heard a horrible racket coming from the cruise control motor (which has never worked previously) and so I disconnected its connection. But, to the best of my knowledge there is simply no way the cruise control can have an effect on the brakes, which are an isolated and closed hydro-mechanical system. I got in the car to test the brakes and they worked just fine. I drove home (about 3.5 miles of stop and go) with no further problems.

I'm at a complete loss to explain this unless of course I have a caliper sticking and the overheating caused some fade. I just don't know.

Has anybody else experienced anything similar?

Car is an '82 300CD.

 

To top it off, I drove to Chicago and back Thurs/Fri (150 miles each way) with no recurrence. This is rather odd in my opinion. The only thing I can think of is perhaps a piece of debris loose in the master cylinder that would have prevented intake of fluid at one point when the pedal was released, but even still that should only have affected two of the wheels and wouldn't totally account for the radical pedal drop.

 

Driving home from work in my 81 300SD in stop and go traffic with no problems at all. Stopped at my favorite Mercedes Garage for a totally unrelated issue (leaking fuel tank) to talk to them about the tank and they wanted me to bring it back later in the week. I was in their waiting room for about 15 minutes at the most, got in my car started it up and started to back out of the parking place and the brake pedal went all the way to the floor and wouldn't stop the car. Cranked her hard to avoid wiping out 3 cars parked behind me in the next row and in panic mode shoved the shifter to park just to hear clicking sound and she kept on sailing backwards towards a busy street. Finally got some sense and hit the emergency brake with my foot and came to a stop. With lots of beads of sweat and a few deep breaths I got my composure back. Looked at all the vacuum lines (my first suspicion) and all were securely connected. Billy (part owner of the shop) came out and checked as well and checked master cylinder (full) and he tried the brakes. Now they were hard full up but didn't want to stop the car easily. Drove forward and backed up multiple times and pumped the brake pedal till all seemed normal. Brakes just started working normal and no clue as to what happened. Billy couldn't explain it either. Drove her home with no incident, all seemingly normal. Any Ideas out there, and did you ever find out your dilemma Cycleboy?

 

I did. It was a warped rotor and a dragging caliper that caused intermittent heat fade.

 

Interesting! Well at least you know. I'll have Billy at Eurotech check on that for me when I take it back for my tank leak. Thanks a lot for that quick reply. Have a safe and enjoyable ride.
Buck