I will add the following:

If you have access to compressed air, blow out the connection where the nut goes over the line. Soak down with PB Blaster, etc (as stated, WD-40 is useless for this), allow to stand a day or two. Blow out with compressed air again (use eye protection!!).

If you have access, get two hammers in there. Place one on the nut, brace it, and give the fitting a good thump with the other, dead square. You must be carefull not to smash the nut flats, but you want to give it a good whack. Blow out again (the idea is to get all the road dirt out of the space between the nut at line, so it won't jam up when you turn the nut), soak down again with PB Blaster, Kroil, etc. Using ONLY line wrenches (the kind with five large flats that fit over the line), you should be able to crack it loose without damage. The normal MB size is 11mm, not in the usual sets, you will have to get it separately.

Don't use vise grips unless you are planning to junk the line anyway, as they always flatten the fittings, making them usually impossible to reassemble leak free.

Note that a bad hose can squirt fluid straight back, the line may be OK.

The good news on all of this is that I have only had one fitting give me much trouble -- a badly corroded one on the 72 220D. Al the rest have come loose nicely, including the four hoses on the 280SE.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!