more of the hated climate control servo

[Wiz3D]

'Tis the season to be cold, even in SoCal ... and, of course, the heat and defrost stopped working on my 78 300d. I have read every thread I can find on this site regarding this problem but I haven't seen what I think I need. More on that later. I do remember, while working at the dealer when these cars were new, that we handed servos out by the dozen. Question is, what do I need to fix first to get back the heat and defrost? I'm saving the AC for a later date.

Little background; heat and def worked fine ... temp wheel adjusted the temp and it blew at the right places and times ... until this spring ... until just after a citric acid flush. Soon after, a rust spot came through the little metal bypass pipe between the head and block. Uh-oh? Then, soon after that, the aux wp quit. Replaced it. Then the blower stopped working. Was just simply, finally dead and I replaced it. I know by feeling the heater core inlets and pipes that no hot water is getting into the h/c or even getting by the servo on the hot water inlet side. Hmmm .... I pulled the servo off and it looks good from the outside EXCEPT for the customary crack in the center body of it. Can I check this thing off of the car? I already know from taking a spare servo apart that I may not be able to get it back together again so I'm reluctant to dismantle this one without a known method or diagram.

Does a crack, which is not leaking water, mean that the servo's shot? There was noticable release of vacuum from hoses (the reservoir) when I unpluged the lines ... so the servo's not a likely source of a vacuum leak at least when the car is shut off.

Secondly, does anyone have, or know where to get a diagram of the innards of one of these things?

Looking at the 123 CD rom, I'm not even sure I can test for what the defect is if I don't have a spinning compressor?? Man, I wish I had my old blue shop manuals back ......

Hunter, Brian and guru's all .... how do I go about fixing this issue PDQ without buying a new servo or an "Unwired" replacement? Funds are scarce as I'm disabled with low vision now ... but I do have a couple of parts cars, a slab to work on, lotsa' tools, time, a fair amount of skill and the will to do it.