I've been listening to Rush since summer of 1990. He has never endorsed anyone in the GOP primaries.
Instead, he waits until the nominee is selected and tries to polish the turd if needed (e.g. Dole, McCain, and Romney).
Notice what Limbaugh has said though during this campaign season.
1. He's acknowledged that he and Trump are at least friendly acquaintances but without endorsing Trump.
2. When asked, he's said Cruz is the most conservative candidate running but without endorsing Cruz.
3. When a viable candidate does or says something conservative, Rush praises him for it but without an endorsement.
You’re right. Rush never endorses, but you can tell who they are building up enventhough they don’t endorse. Rush knows Trump and is more friendly to Trump and does try to be more friendly and impartial unlike Levin.
I do believe he endorsed Pat Buchanan but that was a one and only. He refuses to endorse anyone today but he has in fact raved about the methods Trump is using and marveling that nothing he says seems to hurt him.
I get the distinct impression that he is a Cruz man but will accept Trump. I think it saddens him that none of the non-GOPe people have learned anything by watching Trump's success. I have listened to Rush since his first week.