Last post was probably a little TL;DR, so I'll be a little more brief:
Frau Frankenbenz is a 1983 240D body with a 1984 300SD engine in front of a 1981 240D manual transmission and a 2.88 diff. Vacuum and electrical are hybridized between the two. She ran normally for nearly 4 years with no issues on shutoff, then was benched for 8+/- years. The problem: engine will not shut off with key when brake is depressed after moving under her own power. Also, brake pedal will stiffen up at the end of the braking push. Engine running, hot or cold, without having moved will shut off with key. Engine will always shut off if brake is not depressed. What I've done so far:

-Tested vacuum with gauge from pump - 20-22" hg.
-Checked the check valve on top of vacuum pump - is intact and working
-Replaced the o-ring b/t booster and master cylinder
-Replaced the booster hose.
-I have gone over all the connections and replaced any rubber that seems old, stretched, or dry rotted, and verified that everything is connected that should be.
-Have systematically removed the door lock circuit (locks work after replacing a cracked line), the AC circuit, the EGR circuit, with no change in outcome.

Any suggestions are welcome. She's ready to roll otherwise - just need this one thing sorted out! Thanks in advance, without this forum, Frau Frankenbenz would be a big brown brick.

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Frau Frankenbenz: 1983 240D manual w/ 1984 300SD engine; 81 240D tranny; 2.88 diff. 317,XXX miles.

2004 VW Passat TDI Wagon: Offline awaiting an oil pump balance shaft delete

1978 International Scout 4 Cyl all stock: Awaiting exhaust repair

2005 Ford E350 Diesel: Getting everything from point A to B without complaint

Is the check valve in the booster line functioning? Does the booster pump up to full vacuum, or does it have a leaking diaphragm?

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Disconnect the small vacuum lines from the main brake booster line and connect a vacuum Gauge to one of the nipples and cap off the other nipple (if you have 2 little nipples on you vacuum line).


Apply the brakes as you have written and watch the vacuum gauge to see what happens.


After that it is likely you need to test the vacuum brake booster itself to see about that previously mentioned diaphragm leak.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the replies.

Booster line is a brand new Mercedes part. Air test shows that the check valve does work correctly.

Not sure if I did this right, but I took the old booster line and cut off the check valve (nipples were broken anyway) - attached the flared union of the old booster line to the booster and used the largest mightyvac wedge fitting in the hose to attempt to test it with the engine off. It did not get to or hold vacuum. Does that tell me the booster is bad? Or should I attempt to do this in a different way?

When I have the gauge attached to one of the nipples with the other blocked off, with the engine running the vacuum drops dramatically when I depress the brake. Is this what it should do? Or does this tell me the booster is bad?

Thanks in advance for your help!

__________________
Frau Frankenbenz: 1983 240D manual w/ 1984 300SD engine; 81 240D tranny; 2.88 diff. 317,XXX miles.

2004 VW Passat TDI Wagon: Offline awaiting an oil pump balance shaft delete

1978 International Scout 4 Cyl all stock: Awaiting exhaust repair

2005 Ford E350 Diesel: Getting everything from point A to B without complaint

This is the tool i use to check the vacuum pump and brake booster. The metal part came from an old vacuum line and the nipple from a vw jetta egr. I think the hose is 3/8" ID. Works good.

Attached Thumbnails

 

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

The vacuum will drop immediately when pressing the brake pedal. This why you have that big fat line running up to it and all the accessories such as HVAC pods are tied into the vacuum reservoir.

If your vacuum system is healthy, you should see the vacuum climb right back up. If it's taking it's dear sweet time, or if it isn't pumping back up at all, you may have a bad booster OR a weak vacuum pump.

If you keep the pedal down, see if the vacuum pumps back up. If it pumps up to the normal 20-22", but slowly, you probably have a problem with the pump. If it never pumps up, you probably have a tear in the booster diaphragm.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

A check on brake booster is, to push on the brake pedal [no engine running ] till all vacuum in the booster is empty, the pedal will then go down on to the floor .Now with foot on the pedal and the pedal on the floor, you start the engine .If booster is good it will lift your foot if it is a good booster .

Umm.......no it won't.

The vacuum boost is to help you PUSH the pedal, not lift it. If you keep stomping the pedal without a vacuum source, the pedal will get harder and harder to push until it becomes very firm. If you then start the car with FIRM pressure on the pedal, you should feel it SINK as the vacuum helps boost your foot pressure.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Diseasel300 is correct.

You're wrong ~ this is the standard test for all vacuum assisted brakes .

Go try it and you'll see .

It sounds like the booster is Kaput .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

In any case, did the test with the vacuum gauge. Pressed pedal, kept it down. Vacuum went down to ~3.5" hg, did not recover. So, by all accounts, booster is bad. Calling the parts store now to find one. Thanks for all your help!

Booster is definitely toast. Probably has a tear in the diaphragm.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)