Is this normal for a Mity Vac pump?

[jbach36]

I bought a used Mity Vac, never had one before. It's an old plastic model. If I put my finger over the hose, then squeeze the pump handle completely and fast (in one swift motion), the guage reads about 18 pounds of pressure. If I pump it completely but slow (taking 1 or 2 seconds to complete the motion), I get maybe 10 or 12 pounds of pressure.

I don't know if the unit is messed up, or if on a quick, one motion squeeze of the handle, there's simply more force, therefore giving me a higher reading. Either way are complete squeezes of the handle, so I'm thinking it shouldn't matter for that short period of time. I know after 4-5 seconds, you start to lose vac.

What d'ya think?

jeff