How much air in the Pre Filter is ok??
 

Every MB diesel I've owned I've noticed that no matter how much priming I do there is never solid fuel in the prefilter. There is about 90% fuel and 10% air. Is this normal? If not how do you get it to solid fuel. I've tried all I can think of... Held the filter upright so the air would rise to the top as I reved the engine. Unplugged the output of the filter and let fuel run out of it (again in the upright possition as to eliminate air pockets). But can't seem to get that 10% air out of it.

Wondering if this is ok. The 603 engine in my 87 300d seems to sputter up a hill. so I figured this maybe could be a cause.

A 1/4-1/2" air bubble is a standard feature. Nothing you can do about it.