Actually, I haven’t had time or the weather nice enough to try the following, but it purports to be THE ANSWER. I found this on a chevy site and then again on another site. At the end he says not to use any flame or hot method near fuel related items. Lastly, I have tried the paraffin trick with a rusted bolt and it does work:
"First of all I am going to share some of my father’s favorite tricks for removing rusted fittings or bolts that I have used during my career in the automotive industry. (My dad would be so excited his tricks are actually being published.) Since you have already tried the penetrating oil method and bloodied your knuckles with no luck. Let’s try a few other methods.
First, use some Brakleen to remove all the oils and contaminants from the component and use compressed air to blow it dry. If you do not have compressed air let it dry naturally. The penetrating oil will prevent the next step from doing its job.
Spray some water on the rusty component and let it sit for about 15 minutes. (Yes, water.)
The water activates the rust, and if you think about it that’s what made it in the first place. Water, in my experience, works better on some rusty components than rust penetrants.
After 15 minutes give the rusty nut or fitting several sharp raps using a hammer and a flat punch, reapply water and let it sit for about 15 minutes.
Try to loosen the component. I recommend you not use constant force but rather blunt impact/force. This is an important step when attempting to loosen any stubborn bolts. Strike the wrench to try and loosen the fastener.
If the bolt becomes loose but it is still tight try to rock the bolt. Slowly work the bolt or nut back and forth. Take your wrench and loosen the fastener until you feel tension behind it. Then go back the other way until you feel tension. As the bolt or nut becomes lose you want to spray some penetrating oil on the component to help the process.
If the water does not work there’re a few other old-time mechanics tricks my father taught me. One involves using straight vinegar or brake fluid, or even a 50/50 mix of transmission fluid and acetone, but be careful of the fumes acetone can produce. (Note: Do not let any of these products get on any painted surface as they all will damage the paint.)
Spray the stuck hardware and let it sit. Then again use the blunt impact (force) method and try to remove the stuck bolt or fitting.
There’s always the old heating and cooling method. The expansion and contraction that comes with heating and cooling can break a fastener loose from rust. With a handheld propane torch, heat-up the bolt head until it turns red hot and while it’s still red-hot, squirt it with water.
Repeat the heating and cooling process several times with the torch and water. When using the heating and cooling method be sure to follow proper safety procedures. Wear proper safety attire, including welder’s gloves and safety goggles and have a fire extinguisher within arm’s reach.
Do not use this method near any fuel source, including fuel lines, painted surfaces, or near anything that could cause an explosion or fire. If needed, when the bolt is hot you can apply paraffin wax to help coat the threads of the bolt. Make sure the wax is able to run down into the threads. Remember, too much heat can destroy the temper of a fastener so after heating any fasteners they should be replaced."