84 300 SD Won't shut off - brakes rock hard
 

Earlier this afternoon the brakes went rock hard and the engine won't shut off. I realize this is a vacuum related problem. Any suggestions where to start...?

So far I have checked...the shut off on the pump tests fine with a mighty vac (engine stops under vacuum), the door locks work fine. I put a guage on the vacuum pump and its pulling down slowly (nothing major, but it pulls a vacuum).

I'm leaning toward the brake booster. How does one test the booster, and how hard is it to replace? I see the OEM is pricy...

Thank you for the help!

I did block off the booster line (with the engine running and the key off). The engine continued to run, which puzzles me as I thought the vacuum shut off would have worked (I did test the shut off). So, the booster line disconnected and the car still will not shut off.

How much vacuum should the pump be pulling? I get a slow draw down, but certainly nothing major. I would think the pump either work or it doesn't, right?

"I would think the pump either would work or doesn't."

There are Folks who troubleshoot by replacing parts until they replace the one that has the problem and are those Folks who troubleshoot and find the no good part and replace that part.

Troubleshooting by replacing parts requires a lot of luck or Money or both.
Troubleshooting to find out exactly what is wrong requires you to learn how things work and how to test them.

Besides the Vacuum Shutoff Valve on the Fuel Injection Pump there is a Vacuum Valve on the Steering Wheel Lock to the side of the Instrument Cluster. That Valve or the Hoses to that Valve could also be no good.

Insufficient Vacuum = no Engine Shutoff.

Test the Vacuum Pump with a Vacuum Gauge directly at the Vacuum Pump; I believe you need at least 21 inches of vacuum.
If the Vacuum is no good unscrew the Check Valve and clean it with something like WD40 and inspect it to it is gunked up in a manner that would prevent it from sealing.
Put it back on the Vacuum Pump and retest the Vacuum Pump. If still no good shop for a new Vacuum Pump.

To test the Vacuum Shutoff Valve on the Fuel Injection Pump you can use a Vacuum Tester with a hand pump and Gauge on it (often called a Mighty Vac) connected to only the Vacuum Shutoff Valve or you can simply suck on a Hose connected to there. If your Engine shuts off the Valve is good.

Hook a Vacuum Gauge to the original Hose that connected to the Fuel Injection Pump Shutoff Valve and turn the Key off and see if it gets Vacuum. If it does not it could be something wrong in the Steering Wheel Lock Vacuum valve.

Problem solved.....Check valve on the vacuum pump was bad. It actually broke internally. Easy fix...