You do not mention whether you removed the pump element check valve to allow fuel to flow.
We quit drip timimg MB diesels when the mechanical pumps started coming around in the late 70's. We use the high pressure method now which is exactly the same idea only a high pressure pump pumps the fuel through the valve. We bought this tool when we started having trouble getting flow with the mechanical pumps.
The concept to drip timing goes like this: The pump plunger goes down, uncovering the fuel feed hole, the plunger/piston goes up and closes the feed hole. At that time the fuel starts being pushed through the line and out the injector. This point is called "beginning of delivery. In the early pumps where the pump element check valve was removed, gravity caused the fuel to flow through the pump into the pump piston chamber and out via the missing check valve through the drip tube. As long as the piston was down and there was fuel in the filter the fuel flowed. As the pump was turned the piston eventually rose and when the inlet hole started to be closed the stream of fuel through the drip tube slowed untill no flow was attained with the hole totally closed. The one drip per time interval was the measure of how closed you were. If you were "no flow" there wasn't an exact position as no flow would continue till the piston again came that far down.
The point to all this is that we have always had problems getting flow from the pumps with non removable check valves. Your flow has to be distinct or you won't get the measurement you are looking for.
Bosch Master, ASE Master, L1
33 years MB technician