Your heavy handed. In 20 years of cycling and wrenching, I've never broken off a presta valve. Yet, two days ago, I saw a guy at a bike shop whining about breaking off stems, because he was so strong. You don't use brute force, but a firm technique. Heck, you don't really need to pull the pump head off the stem, as releasing the catch will cause the back pressure to push the head off.