My bike is a 2009 with factory cast HD rims. They have the factory metal valve stems with metal caps that have the sealing "O" rings inside. I have new Metzeler ME880 Marathon tubeless tires on front and rear. Without fail, both tires will loose about 1 PSI of air pressure per week. I check my tire pressure maybe once a month and every time I check them, they are down a good 4 psi in each. I always check and fill them when the tires are stone cold. Wish I could remember if the factory Dunlops that came on the bike did this, but I can't. But the last set of Metzelers also did the same thing. Before I go crazy looking for a leak, would this be considered normal? Tks!