Had new tires installed (OEM'S) about a week ago. The first time I checked for tire pressure, all the air came out of the valve stem in the rear tire. It is a 90 degree steel stem. I checked to make sure the core was tight and refilled to 41 llbs, and then rechecked and all was fine. I took a ride, but decided afterwards to return to my dealer and have a new core installed. A few days later I re-checked my pressure and the same thing happened. I refilled to 41 llbs, and rechecked the pressure with no problems. Then I took another ride with no problems. Has anything like this happened to any of you? and if so, what was causing the problem? The dealer said that he often has valve cores that sometimes stick and let the air out, but in 40 years of riding this is a first for me. Anyone have any ideas??? Should I try for another valve core??