I had a chance to check the tongue and coupler more carefully this evening. It does seem that the coupler is all the way out. The shock absorber seems to be fully extended - - - it can only compress as the coupler is pressed back inside the tongue (thereby activating the brakes.)
Interestingly, I was just checking everything over when I noticed that the plastic cap for the brake fluid reservoir on the tongue was very loose - - - in fact, it came off easily with just my fingers. I made sure there was still brake fluid in the reservoir then carefully tightened it down a little bit with a wrench. I doubt this would cause the brakes to rub as I backed in by hand, but I thought it was curious.
Next, I went to each side of the trailer and tried to rock the trailer back and forth. On the port side, there was no apparent resistance. On the starboard side, however, there was a definite rubbing sound and resistance. Now, I'm wondering if there is something else that I need to do.
Any ideas? I've worked on many vehicles in my lifetime and have done a variety of tasks, but I've never done anything with brakes. Do I need to bleed the lines? Is there an adjustment I can do?
For the record, this trailer is from 2004, but the boat was kept on the lake and in storage, so it looks like it has only a few hundred miles in it. The trailer has likely sat for awhile.
186,000 miles per second - - - not just a good idea, it's the law.