Oh, yes they have. And to the degree it’s lessened I give credit to Fox. The tremendous popularity and ratings Fox has acheived has apparently opened the eyes of the other networks to the fact that whether they can recognize liberal bias in themselves or not, the American public certainly does, and they resent it and flock to a network such as Fox as a result.
Have you read Bernard Goldberg’s book? He does an excellent job of pointing out how liberal bias manifests itself and the reasons for it, including the fact that those engaging in it are unaware of it themselves. In the eyes of the media, there are normal, good-hearted civilized people such as themselves, and then there are right-wing crackpots! I forget the exact figures but IIRC he said that a poll of some 80 or so journalists revealed that 78 of them voted Democrat in the previous election. And their broadcasts reflect this bias.