I am not absolutely sure of the timing but I believe that up to the end of the Mexican Wars, the United States had no admirals, only a highest rank of commodore.

I have been told that the rank of Admiral was instituted in order to bring our system into agreement with those of the other sovereign nations with whom we exchanged salutes. In other words, our guys were interested in face and were coming up short in their dealings with other navies.

It would have also been disadvantageous to try to negotiate as an equal with someone whose nominal rank was higher than yours, no matter what the egalitarian desires of Congress were.