vacuum in water separator filter

I have a 2005 Columbia with a Mercedes and it has a davco fuel pro filter 382 besides the regular filter. There is no ck valve in the fuel tank that I found but there is one in the Davco filter inlet. I am having problems once and awhile with a vacuum in the Davco filter which then starves the motor for fuel and we all know what happens then. I blew out the fuel line and got nothing out. I tried pressurizing the tank and got no fuel out of the inlet line so am I missing a ck valve some where or is it something else.

 

Answered my own question, we changed fuel tanks a couple of months ago and thought we had the replacement tank clean, used a small camera and found what was plugging the line, A F###KING DEAD MOUSE