If the gasket isn't broken and the pump mating surface was clean, then the reason for the leak is the 3 Allen bolts not being tight enough. Did you use an inch torque wrench (89" lbs) to tighten those bolts? A lot of the pros scoff at using them for these types of jobs. But for those of us who don't do this work every day, it's a big help to not be guessing what "just snug them up" means. It's likely the gasket could be used again, but given the cost of a new one, I would do that instead.