How to free a "frozen" joint?

I have a crack in one of the (brass?) isolation ball valves that is connected to my pressure vacumn breaker. I want to replace the valve without damaging the PVB, but I can't break the joint free using "brute force". I haven't used WD-40 on it yet, so that's my next step. If that fails, should I heat the joint with a torch? Any other suggestions?
Thanks for any help - fish

"frozen Joint"

If all else fails use heat...........

You will get much better results using a penetrating oil like PB Blaster vs WD-40.

Use bigger wrenches.

Heat did the trick

Thanks for the suggestions!

fish