I haven't ridden for about 10 days due to a combination of business travel and weather. Normally inflate my tires to about 120 psi due to my current 215 lb. weight. The rear tire was still very hard (>100 psi). The front tire was flat. Tubes are different brands since the rear is original and the front was recently replaced due to a flat. Would this much variation be expected in the leak down rate of different tube brands or should I expect a leak. Would the difference more like be due to the valuve design?