85 300 SD....very hard to start

This starting problem has only developed this winter, so I am not sure if it will start better when we get warm spring weather or not. But It has been very hard to start.... if the car has been sitting unused overnight. I was often cranking until the battery died, and then I could finally get it to start with a jump (even then taking a long time). There was one day that it wouldn't start even with a jump, and priming it manually with the hand pump got it to start...not on the first try after priming, but pretty quick. But the next morning, priming did not help. we had to crank the engine many times with a battery jump even after priming the fuel pump. All of the glow plugs were working except one. So I took the car to my mechanic, and he replaced the one bad glow plug, and replaced two bad fuel lines. You could not see any fuel leaking from them, but they were swelling up, and were always damp (with fuel, I assume). He also said the battery failed the load check, so I got a new 750 CCA battery for Mercedes installed. Got the car back yesterday. Started first try leaving the garage (warm afternoon). This morning it is near freezing, and although the engine does turn over a lot peppier now, I still had to crank it about 12 times before it would start. Dragged the charge down on the new battery a lot, but not nearly to "dead". Since all the glow plugs are working (all have 12 V at the terminal), and the engine is being cranked plenty fast, what can it be other than a fuel supply problem? My mechanic isn't equipped to do a lot on diesels, so should should I take it to a diesel specialist garage and get them to do a pressure check on the fuel injection rail? I can't see it being bad fuel injectors, as sometimes (other than when it has been sitting all night long), it will start up first time every time, as you drive around on errands. A clogged injector is clogged all the time, right? Worse yet, the mechanic replaced the little clear (actually blackened from fuel impurities) bowl on the fuel line going to the fuel pump& hand primer pump assy. The replacements are for some stupid reason opaque white, so I cannot see what is happening with the fuel AT ALL now! Unless there is high pressure there, could I just get some in-line fuel filter with a clear housing and install it in place of the new one?