car runs with key off

Let me start by saying that this web site is brilliant. I am new to the Mercedes world but I have been a VW mechanic/restorer for many years. I just bought a 1970 280SEL and when I turn the ignition off the car still runs. This problem only happens when I leave the fifth fuse in. if i pull the fuel pump fuse during this key off running condition the car will die after about 10 seconds. if i leave the fifth fuse out the key will function properly. I am also looking for a high quality service manul for this car with color wiring diagrams. I have only two more questions for you. First is does the mechanical fuel injection use a high pressure fuel pump and function similar to a L-jetronic system, or does it use low pressure to the fuel distributor and then the fuel distributor pumps the pressure. and last can I convert the transistorised ignition to standard points without effecting the injection system? Thank you for your time and have a great day.

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Check the brake fluid reservoir & make sure that the warning lights don't stay on, feed-back thru this system can keep the ignition circuit powered up.
The fuel system is low pressure from pump, only 14-20 psi. The real injection pressure comes from injection pump.