Removing a stuck/frozen compression fitting
I have found WD-40 to be a lousy penetrating oil. Liquid Wrench, Sea Foam or PB Blaster are much better. Experience has shown me that none of these are particularly effective on aluminum, I am not sure of their effectiveness on brass to brass. But it is worth a try. My usual method with ferrous fittings is to wrap a small strip of rag around the fitting and soak it for several days. Not sure if it works for brass but I've had good luck with  household strength ammonia on aluminum parts. It might be worth a try.