How to check and replace a Schrader valve on a boiler 

I always put a small amount of silicone grease on the valve before recharging the vessel, that way the grease Coates the cores o ring.

Silicon grease can clog the valve up, I use wd40 and find it better.

And if the valve is seized and the top shears off as sometimes happens in older boilers...tap a small Allen key into the remains of the valve and gently wind it out. It saves a lot of grief if you find yourself having an "Oh Bother" moment.