That is an impressive trailer build.

Just one question; Is the airline coupling that you used a Schrader type male & female coupling? With Schrader airline couplings you have to twist the body a 1/4 turn to release the two halves ...... a much better, safer and more secure system than using the normal PCL "Pull back" type quick release airline fittings.

I use Schrader couplings exclusively up at the workshop on my compressed air system as they are less likely to be accidentally released as the airlines are dragged across the floor, bench and work pieces, etc.