Hard starting when parked up hill

Every time I park the truck in my driveway which has incline truck has hard start problem. Even in summer it will crank for few sec but on flat or downhill it starts right away. Yesterday 18 deg no plug flat surface starts very quick today at home 25deg plugged had to crank it three times to get it started. I am thinking maybe something wrong with fuel lines or a pick-up tube in tanks. I got about quarter fuel in it haven't tried with full tanks anyone had any similar issues?

2002 Pre emissions bk 12.7

 

Check valve on the back of the head is brand new two weeks old. This started happening last year was thinking of draining my devco and check it's check valve just in case. If all else fails I will have to check fuel lines make sure there is no small air leak that drains fuel back.

Just a quick update parked truck downhill yesterday at 5pm just started it fired right away. If I find the problem will give an update

 

Update went out to drain fuel out of devco took cap of and it sucked fuel back to tanks. I new right then it was bad check valve. Took it off it was full of dirt cleaned it and all good now holding fuel without cap