That knob is an idle adjustment that repositions the "zero"set of the throttle linkage. Yes you can disconnect it and the car will run fine. Follow the black coaxial looking cable from the firewall to the throttle linkage just above the injection pump. Notice the steel line cable that extends outward from the cable end. It will attach to the throttle linkage in that linkage assembly mess that make up you throttle linkage. You will be able to see how the cable end is adjusted and disconnect it there. To operate it normally. Slightly depress the throttle. rotate the knob, counterclockwise to its stop. Start the car. the car will isle at a higher than normal RPM. I return it to the full clockwise position once the temp guage comes off the low limit.
What concerns me is the High RPM. The cable only increases the RPM a couple of hundred RPM. BUT if your throttle linkage is screwed up this could present a problem.
Check all the ball and socket joints. They should be be attached but "loose". They stay on by a spring C clip. Place a flat blade screwdrive under one and "pop" it off. This will give you a feel for it. They should all have rotation movement and a small amount of play while attached.
Do all of them look straight, attached, free of movement and are all there??
Vacuum, Yes vacuum is the most important thing in a Mercedes Deisel.
Vacuum runs the brakes as you discovered. It also runs EGR, door locks, AC controls, transmission shift, when and how hard/soft, fuel shutoff.
Your car is a non turbo. It has no power, It is slow compared to a gas car. So it may take some getting used to. I will include a picture of my 240D engine setup. Your will be somewhat simular. Except yours is a 5 cylinder. The throttle linkage is close to yours. So if yours is not identical donn't be allarmed. It should follow the basic setup.
The non turbo uses vacuum to determine shifting and at what point. It goes from the vacuum pump thru the large black line that goes to the booster. Its tapped off the at line and routs thru the injection pump vacuum control vavle on the rear top of the IP, thru the white air switch on the valve cover ( it is under the black plastic cover) and to the transmission modulator valve.
Your indication of loosing the brakes is not good. Either the booster failed or the vacuum pump failed, or a vacuum line came loose while driving. Owning a mityvac, vacuum guage is almost a necesity with the Mercedes. Whith this you can measure the output of the vacuum pump, the condition of the booster, the condition of the door locks, Ac elements, etc.
Look at the picture If included and see if all your throttle linkages are there. Are all the vacuum lines in good order and producing a tight seal. Older cars, the rubber 2, 3 and 4 way connectors cam allow vacuum to leak. You must ensure your vacuum system is in good shape prior to solving and other vacuum problems.
Heres a link to the vacuum diagrams I posted. find the one that is closest to your car.
http://www.peachparts.com/shopforum/showthread.php?threadid=91525&highlight=vacuum+diagrams
Dave