Product review (5 stars): so I hammered it out on an anvil first to make it "flat" and it worked great when installed
When I read the 1-star reviews about leaking, I was nervous; so I checked out the brass flange on the 90-degree fitting VERY carefully before installing.  One had a slight "wave" in the flare, so I hammered it out on an anvil first to make it "flat" and it worked great when installed.  One thing a plumber told me which I did was only tighten 1/4 to 1/3 of a turn with the wrench [after hand-tight]--many directions say 1/2 to 2/3 turn.  This reduces deforming the rubber washer which actually causes leaks when over-tightened.