Everything tightened up correctly? HPFP fittings, FPR etc etc?
As above, you may have pinched one or more of the low pressure side O rings on reassembly. Maybe arm yourself with some fresh ones before going back in there again and also good advice from LC5F, lube the O rings with Vas, KY, Olive oil, butter, silicon grease, what ever takes your fancy. Shafts don't like going in dry.