Be aware that it is virtually impossible to get all the air out of the prefilter so it is normal to see a bit of air in there, you just don't want too much. If you have a lot of engine vibration the fuel may be foaming a bit and look like bubbles. Most importantly you shouldn't be able to see any air at all in the hard, transparent fuel lines running fron the secondary filter to the IP.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold