I just pinched this line to see if my lift pump was going out. I just wanted to make sure it was the correct line. Is the one circled the fuel return?

Driver's Side:






After I pinched it, the fuel pressure dropped to about 6 psi and kept slowly decreasing. So I immediately unpinched it and it went back up again.

I just replaced my fuel filter and cleaned the pre-filter (it was pretty clean). After each one, I used the primer to fill up the filters, and for the first 15 mins. or so driving afterwards the fuel pressure comes up to about 20/25 during idle/driving and the truck drives great. Then after a while it drops back down to 16/20 during idle/driving and the truck feels a little sluggish, maybe it's just me.

I was going to mod the overflow valve but I thought I better find the problem if there is one. Does this sound like my lift pump is going out?

 

I think that's the feed line.
Return in red below.