rims leaking air

i have a 2001 gtp special edition coupe,purhased new ,has only36k miles,car is taken care of better than most,i work at a bmw dealership here in new jersey,vehicle is garaged all the time and is professionally maintained by me,the special edition model has 15 spoke rim in chrome ,whats happening is in the valve stem area,there is corssion forming and the valve stem is not seating as it should ,causing loss of air pressure in tire,i keep the tires at 40 psi all the way around ,it rides alittle stiff but if i never did that i would not have noticed the the corrsion ,cause i kept on getting low tire pressure in 3 tires,vehicle has 3 bad rims ,my advise to you is if you have a special edition check your rims for corrsion ,just look innthe valve stem area,move valve stem around and look for corrsion, said that i work at a bmw dealership i can easily put vehicle on lift and actually perform maintaince as it should be done, and not worrying about sense less mods to go nowhere fast