Ran my 300CD for the first time in ~7 years...
 

Bought my 300CD in CA, drove it cross country, drove it quite a bit around on road trips, etc., then got caught up in figuring out window regulators, parked it in my garage, kids came and... Now Im trying to get it back going.

Had changed out the coolant to G-05...

Diagnosed a leaky schrader, so changed it...

https://i.imgur.com/R1ndlaSl.jpg

Got tires mounted on my CalChrome Clear'ed 15" Bundts:

https://i.imgur.com/4zOWr5Dl.jpg

And went for a drive...

The AC charge is insufficient, and it was 86 out, so it was OK but not cool in the cabin. I can fix that.

There is a sound I hear at 40-50 but nowhere else, will need another thread on that...

There is no heat. Tried running the car with defrost on when filling, no dice. I dont particularly care about having heat besides wanting active chemistry in the heater core.

So the issue...

Driving along, under harder braking, like a more rapid stop at a red light, it felt like if I applied the brakes it was OK, but let off there was a front end clunk. Like something is loose in the front end.

Now remember that when I parked it, Id drive the car anywhere. Literally did. Ran great, nice AC, etc. I did stuff to it, but never encountered this. Almost feels like the suspension is shifting forward and backwards a tiny bit when doing this braking action...

So I got home and checked the wheels - left front wasnt able to be touched. Immediately jacked the car up, could barely touch the lugs, got them off with my impact. Got out the IR camera.

https://i.imgur.com/Auqb9Bwl.jpg

https://i.imgur.com/7dIsiarl.jpg

(ouch!)

https://i.imgur.com/9QnxsVGl.jpg

This is it in real life. Nothing could be touched 15 minutes after removing the wheel.

https://i.imgur.com/fgDmigSl.jpg

For reference on the other side...

https://i.imgur.com/9UzzRozl.jpg

Which still seems a bit hot to me...

https://i.imgur.com/JK8XxKxl.jpg

I know, not the exact same spot on the rotor, but it was much cooler overall.

So... I guess it means seized caliper, and need to re-do the entire brake system. Not a big deal.

But my questions are now:

1) When braking, it felt like something in the front suspension was shifting back and forth very, very slightly. Maybe it was a seizing of the pads to the rotor? nothing on the front is loose, I had rebuilt most of the front suspension, and the steering on this car is tighter and better than any W123 or 126 Ive ever driven, including my 240D with 73k miles. Is there something else I should check given that the typical play and shake tests yield nothing, and it only happens after a hard stop?

2) We drove all over doing errands. Probably 30 miles, six startup and shutdowns, etc. All was perfect. But the last leg, the last 2 miles just in town, slow to get home, the braking got softer in the pedal, and after I pulled up in my driveway and tried to shut the car off, it hesitated to shut down. Im not sure that the continuously softer pedal and the difficulty shutting down were coincidental. But Im not sure the seized/hot caliper is the same error as if there is a brake booster/vac issue. Any thoughts?

Thank you for any recommendations!

When I encountered something similar, re-torque of the caliper bolts seemed to resolve the "shift".

Pull off the main vacuum line connection that screws onto the vacuum pump. Fish around in there and see if any debris or the vacuum check valve broke off. That can clog up and reduce vacuum.

Man that's HOT, you think the bearings and grease are ok?

That brake rotor is such a pretty shade of blue.....

Time to look for restriction in fluid back flow, might well be a hose .

Don't forget to re pack the wheel bearings .

Nice looking Coupe ! .

Aren’t those single use bolts? I will give that a try for diagnosis purposes.


I had to move cars in the driveway so I pulled it out and then back in. The booster/pedal is still very soft, feels more like my Chevy pickup then any w123 I’ve owned. Not clear what that means... but the engine did shut off as normal when I did that vehicle exchange. So I suspect that it may have been my pumping the pedal that affected shutdown due to the firm, pumping motion Imapplied to see if I could get my pedal harder again.

So it begs the question, what made my brake pedal go soft and stay that way?

My vote is for a stuck brake piston - though it is always best to check fixings / mountings / rubber parts if a car has been sitting for a while.

Clunk braking is usually guide rod bushing/ mount

I know. That’s my consideration/concern.

Not really sure how it could go bad while sitting, but that will be something to pursue. I’d suspect that if one brake is dragging like it is, perhaps the tire front end has a bit of misalignment or pull on it due to that, which might then make a part that is marginal behave worse. Not looking forward to that job if it is required...

I’m going to re do all the brakes and go from there when I have time...

I agree that the heat problem is probably a sticking caliper. Had the same problem on my '81 300SD; changed it out and problem resolved. Before that I could take a short drive and the wheel would heat up so much I could not touch it. Other three wheels would be fine.

Some of your difficulties are probably directly related to that stuck caliper.

Yes, the caliper is my first job to fix, potentially before I even do a real brake job.

I’m not quite seeing how I can get the clunking feeling upon brake pedal release without something else in the front suspension being off, but not sure what it could be either.

Work Smart
 

And do the entire job in one go, it'll be faster, easier and cheaper plus a better job in the end .