+1 heat
Soak it with penetrating oil first, apply daily for a week, can try to remove with a strap wrench periodically.
If and wen you are ready to write off the cap get your Dremel out and cut some shallow reliefs in the threaded area or just grab the cap with a stout set of water pump pliers; the big brother to the vice-grip family. If you have the skill and get to the Dremel stage then simply cut a 1/2" square in the center of the cap and use a 1/2" drive murder bar to spin it off.
If the cap is in the shape necessary to cause this situation the rest of the master can't be far behind, you may want to throw-in-the-towel and get a new or re-manufactured one which will (should) be sleeved to increase the mechanical (hydraulic) advantage.