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.
m

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Martin Ingram
Colorado Springs
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)