Floor pump popping

[diylighter]

I bought an alloy floor pump of all places at Aldi for $8. It has a gauge and dual head. Surprisingly, it has lasted for 2 yrs so far.

However, my 5-year-old got ahold of it the other day and unscrewed the top and pulled out the plunger. I did not see any other parts lying around, and put it back in with the o-ring intact. Now, it makes a popping sound at the bottom of every stroke, and the gauge goes haywire.

Any ideas on what part might be missing?

This sounds like a challenging one to me, so I will be grateful to the wizard that figures this one out. If a pic is needed, I can do that.

Thanks in advance.

[wschruba]

Is the plunger made of leather or plastic, and also, did you grease/oil the o-ring or edge of the plunger?

If the plunger is leather, it may have been deformed, but more likely, the pop is being caused by suction. Lube up the internals and see if it fixes it. [i]Very/[i] light oil. Like, tri-flow light.

[DMKiefer]

Maybe it's missing the plastic/rubber/metal check ball to keep the air from back flowin.

[Slash5]

I agree, sounds like a check valve. Maybe some crud got in the chamber and you can blow it out? Or could be that air can't get into the top of the tube when you start to reverse the stroke?
Take a look here at this video on how a pump works to see what to look for:
How a Bike Pump Works - YouTube