Mike, Do what John said and spray that fitting with penetrating oil. I usually try tighten the valve just half a hair first, then loosen it. I have found that using a 1/4" ratchet and correct size socket is better than using a combination wrench, It gives better control and eliminates rounding off the corners of the valve. You use the wrench when its's time to bleed. If you can get the valve loose, take it out and go to Napa or your favorite good parts store and get NEW valves. If yours breaks off, no big deal,they are probably pited inside anyway. Napa has new ones at a good price. The way I remember, the guy at Napa thought they looked like a cylinder off a chevy of some sort and they did , which was cheaper. He was the kind of partsman that knows his inventory and what was used since the war. WWII. Good Luck, Keith