when you suck on the line does the part move? if it does then the part is good.
Your A/C servo is like a giant watch. my 1980 300TD and I went around this issue for a year before she would work properly- and that was because I had acces to a/c servos in the yards here of south florida. Lithium grease, digital pics and a steady hand are your friends if you intend to repair your current unit. I could find several unit in the yard her that may or may not work- but my cost is $40-$60$- no testing at all- and I have a 30 day return. Finding, pulling, packing, shipping and taking the risk of returning and non payers makes the do it/dont do it value to me in dollars about twice my cost. So take yours apart, slowly, carefully, with pics, and see if you cant fix it- if you are mechanically inclined and have tools. If not, buy a $15 12 volt space heater.