I have a Topeak Joe Blow sport or whatever the blue one is, and it's recently started acting funny. The pump handle sometimes "rebounds" on me when pumping, and the gauge will not read on MTB tires unless I hold the pump handle down when I pump it (it reads fine with 80+ psi road tires).
Any clue what's wrong with it? I hate to toss the whole thing over some small problem.
I suspect condensation buildup may be the cause. I had two cheap pumps blow out the same night. They were both full of condensation inside. My new pump, a Nashbar Orange had water bubbling out of the valve fitting after only two tire inflations. I live where the relative humidity is always high and I suspect this contributes.
What to do? I don't know except disassembling the pump as much as possible and cleaning it up.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Most pumps have either a ball check or a flap check valve in the base to prevent air backflow. Inspect this valve to see if it's rusty or stuck open. Clean as needed and the pump should go back to working properly.