Is there an o-ring on the union?
You probably need to replace the o-ring that is on the bottom of the union. Open the union and look on the hose side of the union for a o-ring, its likely recessed into the plastic. Remove it and take it to the pool store for a replacement. If you know the brand of the union, you can likely find the o-ring online before removing it.
not likely an exact match, but gives you a good idea of what the parts look like: Pentair Union