Installed almost 100 ft. of new 1/4" flexible copper piping for evaporative cooler. Since I could only get the first 50 ft. In 10 ft. Sections, I had to use several brass compression joints, and have a few that leak. Should I be using a flaring tool?

Not if they are compression joints. Retighten them or replace them. Whenever you have a run of copper this long, it is best to use only one piece of tubing or only TWO 50' pieces. Just my own opinion but I would have looked around to find two 50' pieces or one 100' roll. Those joints will haunt you for a long time.

Appreciate your response and will use two 50 ft. pieces.

Good Move (:-)

Sometimes it is hard to tighten the fittings depending on the type copper you used. Was it M, L or K? If necessary, apply a very thin coating of plumbers grease on the compression ring. You will only be able to easily flare M, harder for L.