D4d Starting Prob Have To Pump Prime

[dago]

hi l'm having a problem starting my 2001 diesel most of the time it starts fine but sometimes i have to pump the prime button about 5- 10 times to start it have checked the line and can see no leaks it mostly happens when facing up hill dosent seem to matter if tank full or not seems random, fine for weeks dosent seem to matter how long it's parked for either cheers