Replacing connector beneath kitchen sink- frail one is stuck tight, back!? I need to replace the hot dampen connector for my kitchen sink-...

I need to replace the hot dampen connector for my kitchen sink- I've removed it from the faucet, but the end that attaches to the sea line (by the cut past its sell-by date valve) is stuck tight, I have worked on it beside a wrench until my muscles about give out, I've hosed it down in WD-40 and 3 surrounded by 1 oil, but it doesn't budge. It's be on there for probably 15 years (original beside the house). Any ideas?
Formica Countertops and Stainless Steel Sinks?
Answers:    Shut the hose down off at the biggest valve and replace the angle stop' the valve'.
The overhead lights contained by my Genie Excelerator...
warmness the connector with a boil gun. you can also replace the cutoff valve