Given Diesel fuel is nearly an incompressible fluid, like water, the pump type selected for the very high injection pressures is a positive displacement pump. The injection pump is also a specific type of positive displacement pump known as a reciprocating pump. Under each steel line coming out of the pump and going to an injector (which is actually a pressure relief valve with a mechanically set relief pressure) is a piston in a cylinder that has a mechanical means to change the point in the stroke of the piston where the pump becomes a true positive displacment pump. Up to that point the stroke is "wasted" recirculating any displaced fluid at a much lower pressure, back to the filter and ultimately the tank. After the point where the pump stops recirculating fuel, the pressure in the steel line rapidly rises with very little further stroke, lifting the pressure relief valve (injector), and for the rest of the stroke, all the volume displaced in the cylinder of the pump is squirted out of the injector. The transition from recirculation to positive displacement is controlled by mechanical linkages connected to the pedal under your right foot. At idle, most of the stroke is recirculating, while floored, most of the stroke is in the positive displacement mode.

It should be clear to see why air, a compressible fluid, inside this setup goes next to nowhere - the stroke at wide open throttle moves some very small quantity of fuel per cycle. In an air blocked system, such as after you open the system up and let air in, the stroke of the injection pump never generates enough pressure to open the injector, and the air has to be slowly inched along through the recirculation lines, which operate at a lower pressure.

Mercedes used to provide a separate, hand operated purge pump, which took a much larger stroke and still required enough pumping to give you a blister from grabbing the knurled edge of the pump actuator. Today they just tell you to run the starter, so, I don't think the injection pump is at risk. If you did this daily I am sure it would be an unplanned operating evolution that could prove damaging, but your starter would be the first thing to go. Followed by the battery. And likely the glow plugs.

So, if your car starts and idles normally, I don't think you have an injection pump problem brewing. Unlike pumps with impellers, known as centrifugal pumps, there is no similar tendency for "vaporization," also known as cavitation, that typically is damaging, in a positive displacement pump (the output pressure is typically much higher than the vapor pressure of the liquid being pumped).

I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)