depends on the origin of the bubbles. if the bubbles are coming all the way down the ink lines from the carts, then the leak is further back at the cart. i'd start by trying out a different yellow cart. see if the issue persists. if you don't have another yellow cart, it might be worth having someone push the yellow cart in firmly and at the other end of the printer pull some yellow ink.
also when figuring out the issue, go ahead and pull the yellow damper and pull directly from the damper end. it will help you get a better idea of the amount of vacuum at that damper. also, pull some yellow ink until the damper is 75% full and let the damper dangle. come back about five minutes later to see if the line is holding or the yellow has receded.