I guess if I was having this problem I would solve it the simple way. Take the wheels off and take them to a good tire store and have new steel valve stems installed. I wouldn't want them things on my rig even if I could fix them. just my $.02 worth !!!
Here is an idea. Discount Tire will air your tires up for free. Just stop in there and tell them you needed to check your air pressure on the tires and adjust it as needed. Then let them worry about getting the valve caps off.
Had same issue with my truck, apparently over the winter the metal caps had frozen to the value stems. I ended up using a Dermal Tool and very carefully cutting a grove into the side of the caps, be carefully not to grind into the valve stem. As said previously, you may be able to just grind off the top of the cap and then get it to break loose. Also, be careful, twisting the cap and rotating the stem as it can become a missile.I have a 3300sk, on 2 of my tires, i can't remove my valve caps, they seem to be made out of some type of metal, ( not plastic ). I used Blaster penatrating oil, and pliers, still no luck..Any ideas ??? Thanks Bernie