nipple wobble ,, don't sound good, but two sets with the same prob.can't really answer your problem but you must have a leak somewhere m8,
my advice would be to check your lines again and possibly try your old calipers yet again!!
if i'm right if the nipple is wobbling your getting air back into the system somehow.
this type of prob.. can be a pain in the asre but you sound like you have a fair knowlege so take your time and check it again and you should get there eventually. Not the best answer i know, but i know what you mean!! the easy ones cause the biggest probs,
all the best m8
regards kevin