i'd wait just a minute. that machine is a bit different than most.
in that machine you've got some limitations for damper swapping. doesn't it have the metal damper tubes? in that you've got just two caps. and from the one cap you are getting M and C but no Y... it seems that the cap is operating fine as far as vacuum. the only way you'd able to diagnose is to move the yellow to one of the black head adapter nipples. moving the yellow to cyan or magenta will only tell you one thing, if your adapter filter is very clogged or not because they share the same cap/vacuum. which is unlikely but possible. the fact that you are manually pulling a vacuum from beneath the head from the pump tubes and still not getting yellow it troubling. put the yellow on the cyan or magenta position and see if you can then pull yellow from the pump tube.
also, when you just pulled ink down the yellow line from the end of the damper, did it seem easy under a decent vacuum. ...the same amount of vacuum that he M and C required. or did take more work?
also, also, is the white pump tube on the front of the printer that connects to the capping station attached firmly? instead of first swapping dampers (since the metal tubes will be difficult to work with), you could just as easily swap those two white pump tubes to test that you are getting proper vacuum and not contaminate your print heads. also, if that cap isn't resting firmly and level to the head. it can pull a stronger vacuum on one side than the other. i've seen this. make sure that it is seating firmly against the bottom of the head. in the service menu, there should also be an capping position adjustment to make sure it parks at it's uppermost position.