Take the oring off the pump, you only want one down in the channel. Now take the channel oring out and carefully clean and examine it for no cracks or debris. Now carefully examine and clean the channel bottom surface to make sure there are no burs or debris.
Typically a leak like that is due to a plastic shaving or similar stuck between the oring and mating surface or perhaps a casting flaw in the surface. Use something to run around that base to feel for smoothness.
+1 on the vaseline, many manufacturers use an insoluble grease to hold orings in place and it does also help lubricate the orings to seal better.