Joe Blow pump "bouncing"?
 

My pump has the dreaded bounce. Where you pump but the pressure doesn't go into the tube. What makes that "one way" air flow happen? Ugh. I hate having to ask or not knowing. Of course I've done nothing to change the pump. It worked last week. So I just now took everything apart that comes apart and put it back together. I saw nothing that helped me understand how a pump works. And it still doesn't work. Just bounces. Well, it'll put 40psi in. That's it. I see it read on the dial when I hold the handle down. Then the gauge goes back to zero when I let the handle rise up. Sigh. Of course I also don't know how to describe what is happening, but I'll try the search "pump bounce"?

[UPDATE!: HA! looks like i had the same prob a few yrs ago and posted here. here's the thread with the solution. actually, here's the solution. tho i haven't tried it yet this time! ...don't dismantle the pump guts, especially from the base. unscrew the side knob near the gauge and fish out the spring, maybe lube it, then put it back. done. here's link to more: https://www.bikeforums.net/bicycle-m...blow-pump.html

Presta or Schrader valve stem? If presta, are you loosening the stem and "burping" the tube so pressure can get into it? I have one tube on one of my bikes that seems to have a "sticky" core (presta) that I often have to burp 3 times before the core will let air into the tube. What you're describing sounds very similar. If that's not it, maybe the chuck is faulty. They are replaceable.

I've had this problem before and fixed it. I forget how... I recall it was easy.

The tube is fine. It pumped with another pump. I wish I knew how a pump works. Why does the air now just go in and then back out. What part keeps the air in?

Many floor pumps have what's known as a check valve in their base. A valve that lets the compressed air travel from the cylinder to the hose but stops air from going the other direction. BTW a Presta valve does the same function but at the other end of the hose. Sometimes these check valves can get plugged up or frozen shut (grime, rust). With a Presta tube valve a simple tap of the end of the unthreaded knurled finger nut is usually enough to break loose the stickiness (as mentioned by others). But for the pump base's check valve one has to take the base apart and most current floor pumps are not made to be serviced at this level and replacement parts (even if one could take the base apart) will be unavailable. Most current floor pumps are considered to be a commodity and disposable. Andy

What happens when you put all your weight and muscle into it? If it still bounces back, then a valve isn't opening. Probably the valve in your tire. Did you unscrew it enough or put the chuck on properly?

It might be a stuck valve in the pump. But also, it might be that you have a large bore pump that simply requires more of your weight and muscle to supply enough pressure to open the valves. It's a noticeable progression that as the tire gets more pressure, it takes much more of your effort to push the next strokes worth of air past the check valve and into the tire.

Large bore pumps will let you fill a tire fast. Small bore pumps will let you get to higher pressures easier but take longer to fill a tire.

Joe Blow is a famous pump, been around awhile. Sounds like check valve in base is stuck. Any info on accessing it? I've fussed w mine before and recall the fix was easier than I thought. there are allen bolts under the floor plate and i recall in the end i didn't need to take those off. like, i removed them but it was a waste of time. the access was otherwise. but i forget the details. i figured since it's so popular others would know this glitch. i've used this pump for 10 yrs thick'n'thin. and am used to using bike pumps of all types. i just dont know what's happening inside them.