Are you possibly unscrewing the stem too far when pumping and bending it. It only needs a couple of turns on the nut to allow air in. Some presta stems are very long and fragile when the nut is backed off all the way. It's also not necessary to torque down a lot on the stem nut when tightening. The nut only keeps vibration and bumps from "burping" air out past the seal in the presta valve.