I unscrew the presta valve, put my pump head onto the valve, and flip the clamp. Then, I begin to push down on the pump handle and the air starts flying out of the tube like nobody's business. I can pump air into it, but not at the same rate that air is leaving the tube.

Is my pump defective, or am I really that stupid?

First, the pump may have an adjustment on the head that makes it dual-purpose for both Presta and Schrader valves. Make sure you have it set correctly for your valve type.

Second, you want the pump head fully and squarely seated over the valve. If it's cocked a bit or not positioned low enough to seat on the valve stem, air will escape.

When all is right, there will be no loss in air from the tube as you pump.

**Add: Some Presta valves have removable cores, though most I've seen do not. A removable core can sometimes loosen up and make airing up troublesome until retightened. Since a loose core will allow air to escape even when the valve is closed, this probably isn't your issue.

The problem cannot be duplicated on my tubeless mtb tires.

If you're pump grabs onto the valve too tightly, you can damage the tube when you try to remove the pump head. This is the case with my pump and I need to be very careful when removing it. When I try to pull the head off the valve, there can be enough force on the valve that it partially pulls the valve out of the tube, then the tube leaks and won't hold air.

Then the valve maybe busted. Just switch to a new tube.

Like that on both tires. They hold air otherwise. As soon as i unclamp the pump all is well.

My floor pump can be used with presta or schrader valves by reversing a little fitting inside the head. In addition to the fitting, there is an O-ring in the head. Occasionally part of this O-ring will slip out of its proper position, and the symptoms are the same as you described. Unscrew the cap on the head, check to make sure the fitting is in the proper orientation for presta, and check to see that the O-ring is tucked in properly. On my pump there is a small pin that holds the clamp in place. If this pin slips out partially, the O-ring will slip. If the head on your pump is constructed like the one on my pump, this may help. Good luck.