I had this problem on my '83 after installing a nos lift pump assembly

Took out the fuel pressure relief spring and stretched it to 27mm(from 21mm)

That took out a lot of the shake and things are smooth now

This is after new rack damper, new mounts, and valve adjust

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1983 300CD Turbodiesel

OK, just spent the last 40 mins driving home with no power brakes. I can stop the car but you have to stand on the brake pedal. When it first happened, about 5 minutes after starting to drive I pulled over. Found a blue vac line had come off a base rubber thing with 2 ends going in and 1 out the other side. I'll take pics in a little bit.
I'm guessing the power assist to the brakes has now left town?? Shadiest drive home I've ever had. There was also a vac line (having to do with the ALDA) I reconnected yesterday. That goes from the drivers side of the engine bay, around the back of the engine and into a banjo bolt on the other side. Will take photos of that.

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1982 300CD (daily driver) 186K clicks

In addition, the new main fuel filter is leaking. It was snug and tight when I left this morning, but when I stopped to see about the brakes there was diesel spray below it and I could see a little seeping out from the mount.

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1982 300CD (daily driver) 186K clicks

Update, I tested the vac line to the shut off valve. Drew air through it and the car shut right off. Could this brake issue also be a vac problem? I'm guessing so. I don't have a Mityvac to test things, but can order one on Amazon for delivery tomorrow. This is my only car, so I have to get her up to snuff quick. lol

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1982 300CD (daily driver) 186K clicks

Yes you obviously have vacuum problems. Most likely a leak somewhere.

R&R the current fuel filter. Look for reasons it isn't sealing. Be sure that's where it's leaking and not from a hose or something.

Check your vac pump. Be sure to isolate it because down stream leaks can make it look like the pump isn't providing vac. If good, suspect the booster but disconnect and plug the vacuum systems in the cabin. The lines to plug are near the booster (ie doors, climate control).

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

The fuel filter was leaking between the filter and mount. I removed the crush washer from my 81 and switched the washers out, it seemed to be fine after that.
I ordered a Mityvac 8000 to be delivered today to test the system. I'm just not sure what I should be testing. I looked for any possible cracks in rubber connectors and replaced a few that looked like potential issues. (I'm pirating off the 81 for those too ... not that they're new).
Obviously the brakes are the main issue now. Since there is just that one line from the pump to the brake booster, could a small leak somewhere else in the direct vac system cause the booster not to have enough vac?

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1982 300CD (daily driver) 186K clicks

Brakes and engine shut-off: Vacuum (or lack thereof) is common to both. It is easy to accidentally knock off the vacuum line to the shut-off valve on the injection pump, so check that first. If all the vacuum lines are intact and not leaking, then your vacuum pump may have failed or the brake booster may have failed (or even the seal between master cylinder and the brake booster).

Rough idle: dead motor mounts are very common (there are three, two at the engine and third at the back of the transmission). If you replace them with after-market, odds are good they are now bad.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Max, I checked all the lines .. nothing unconnected and no cracks or dry rubber connectors that I could tell. I drew air through the Shut Off valve tubing and the engine shut right down. I feel like its a line since I was checking other things in that part of the engine bay. Saturday afternoon after replacing air filter bracket and adjusting the damper bolt, the shut off quit working. Sunday the next day the brake booster stopped. Just doesn't seem as likely pump failure as much as operator did something stupid unbeknownst to him.
As far as the motor mounts, I've check them all and all seem in tact and holding while the engine is running. The vibration exactly at 800 rpms only is the weird thing to me. But my experience with this is limited since I've been lucky with both cars over the past 12 years and nothing major going south.

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1982 300CD (daily driver) 186K clicks

Ok, all lines through the firewall tested and hold vacuum. Shut off valve works fine. I pulled the main line to the brake booster, but have no vacuum gauge. The check valve to the pump has the little star thing in it ... I read that means it's ok.
Things are not adding up well. There is an indie near me that maybe can check the vac on the pump. I'll need someone to replace the pump if that's gone.
Anyone else have any ideas? Oddly, shifting isn't "hard" as what I've read in this case. Except downshifting. That'll knock a mollar loose. lol

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1982 300CD (daily driver) 186K clicks

It's the vacuum pump. Thanks for all the help, I really appreciate it! This place is a real gem. Looking forward to rehabbing more over the summer, so I'm sure I'll be back soon with more questions.
Thanks!

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1982 300CD (daily driver) 186K clicks