I just installed a new Michelin tube with a threadless presta valve. When I try pumping the tire, the head of the pump pops off when the PSI reaches about 100. The seal doesn't seem to be gripping the valve tightly when I lock it. I've only used threaded tubes before and never had this problem. Could the seal be worn from the threaded valves? The pump is a Topeak Joe Blow Sport but it's only 3 months old and used twice a week. Is there anyway to fix this? Thanks for the input!