I found that everytime I dismantled the pump for maintenance
and lubrication that it would leak from the top cover upon reassembly. By trial and error I learnt that it wasn't the gasket
but my reassembly technique. Those screws have to be tightened up very slowly with equal pressure across all corners. That means once all screws are in very loosely, tighten up slowly, moving from adjacent screw to adjacent screw. Not so much pressure that you crack the plastic, which I have done to no real ill effect, but still tight.
This always works for me.