Fuel Filter/Pressure Issue

OK, I replaced my fuel filter with the clear type a couple days ago. All seemed well until today, I stopped at the local shop when I noted the car seemed to be fuel starving. We open the hood and the filter was nearly dry. When I revived the engine there was barely a trickle of gas coming from the inlet side.

OK, easy right? We replaced the fuel pump. Car started fine, filter filled about half way up, so I thought good to go and drove home.

Once home I opened the hood again and noted the filter was a little less then half full. I let the car set for 15 minutes and the filter is now BONE dry. WTF????

Couple questions with my setup that is the OEM mechanical pump. Please note I have not identified any evidence of leaking!

1. When the car is off, how much fuel should be in the filter?

2. With the car running how much fuel should I see in the filter?

3. Why would the filter go dry after setting for such a short time? Could this be a gas cap issue?

Please advise!!