What causes leaking between the heat break and the heat block?
A video I recently viewed (YouTube link) regarding installing a hot end assembly references the proper sequence of parts placement. Starting with the nozzle, thread it into the heater block until it is proud of the surface by a half of a millimeter, perhaps slightly more. The heat break is then threaded into the heater block until it contacts the nozzle. If not already in place, attach the heat sink.

The next step is to bring the assembly up to maximum temperature and re-tighten the nozzle assembly, obviously using great care due to the high temp.

The video specified that performing this sequence incorrectly would result in leakage from the assembly, which describes your troubles.