I bought a pump a few months back, haven't really started using it consistently yet, but I've noticed when I pump, it doesn't hold vacuum, or pressure...whatever you want to call it. I'm a beginner at this, so im not sure whats right and whats wrong. I can never get it over like, 12psi of vacuum before it gets a little uncomfertable, but at any vacuum I get it too, it never holds steady. Is this normal, and you have to pump constantly to keep an ideal range of vacuum?