I am in agreement with moedip, the check valve is the most likely issue. Jump the fuel relay or turn the key to 2 three or four times before cranking it if it starts up fine then you have a slow leak back somewhere the cheapest fix is the check valve.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)