Is it common for these to leak? The one on my filter return will leak a drop slowly from either end in that u shaped grove if I throttle the flow at all. Doesn't seem to if I run it wide open. Sometimes it doesn't do it, but of I fiddle with the valve lever at all it will start again. Is it a faulty set?
Not sure what U shape groove you are talking about. Are you talking about the parts that spin when taking them off?
Anyway, I have had mine leak from time to time, usually after a few times of servicing. If you are having the same issue as me, it's a very easy fix. Take a towel or similar and clean out both sides of the quick disconnects. Then take off the o ring and lube it with silicone grease (I use petroleum jelly if I can't find anything else in the garage instead). That seems to work for me.
Clean out the double taps and lube it. Carefully inspect the U-groove area where it drips from to see if there is a hair line crack. When reconnecting the double taps after servicing the canister, if one uses too much pressure/force to screw in the taps, it will break if not being careful.