For two years now I've been using a humble Topeak Joe Blow floor pump with perfect satisfaction. Last week or two, though, one or both of these things happen when I pump up my tires (which I do before almost every ride):
In mid-pump the chock pops off the valve (presta). If that doesn't happen, or before that happens, on the downstroke the gauge moves up normally, but then falls back to lower levels. At best, I keep pumping until it seems like there's enough air, and then pop the chuck off.
My guess is that both these symptoms come from a common cause, but I don't know what. I'd hate to buy a new pump when the old one has been doing fine.
Thanks in advance for all advice and suggestions.