I flew Kraft for a decade. The KPS14 and KPS15 servos were the standard back then. We might think they were good, but they were not even up to the standards of the $10 servos available today, in my opinion. Backl then, at $40 a piece, we thought we couldn't get anything better. I remember testing centering and travel on them. They were off 5-10% on right-left travel, and at least 3% on centering. Compared to the quality servos available today, it's understandable using a 'Y" back then would be no problem. We probably wouldn't have been able to tell the difference. We didn't even have SMT motors in the servos back then, let alone more than a few transistors. No comparison to today's technology, again, in my opinion.
I was just in the process of setting up the radio for a quarter scale plane on the bench, just checking servos. The "Y" was driving two Hitecs 7954s crazy, followed by two Ace 1213s. Separate links 'mated' to second channels completely eliminated all issues and put those servos dead on.
So, if I were you, I'd follow everyone's advice and don't use the 'Y's and if you have any doubt, swap out the offending servo. It's not worth potentially losing your plane.