Just re-read you message. Your ball doesn't firm up with the fuel vent open? But it does with it closed? That's kind of odd. I'll explain that as I ramble.
The firmness you should be feeling comes from the ball pumping fuel to the remaining fuel line and the engine. Being a one-way valve, once pressurised (only slightly) the ball holds that pressure, and you get a firm primer bulb.
If the ball never firms up (with the vent open - the fuel you draw from the fuel tank has to replaced by something) , there are a few possibilities:
1) Fuel bulb check valve is not working, so fuel pumps to motor, and runs back to the tank.
2) Leak in the post-bulb fuel line. Should be pretty obvious, as when you pump the ball, you should see or at least smell the fuel as it pours out.
3) Completely bad pumping mechanism in the bulb. You aren't pumping anything anywhere.
You said yours firms up when the fuel vent is closed. So, assuming everything else is working OK, the fuel tank pulling a slight vacuum is causing the bulb firmness. That, to me, seems backwards. It's odd, because I would have expected the ball to simply stay collapsed.
4) Never mind; got it. Your primer bulb is installed backwards. You're poumping fuel into the tank, rather than from the tank into the motor. Close the vent, and the tank pressurizes, causing the firm bulb.
Turn the primer ball around on the line, and I think you'll be good to go.
Geez; maybe I should have read Erin's message first. Would've saved some typing.