You're overheating the joint and the lumpiness is from the solder going bad
from the heat.
Keep the heat down on the joint by not heating it but rather the metal near
it. You may want to go to a soldering iron rather than the torch on thin
sheetmetal pipes. There is no reason why brass and copper can't be soldered
together as the brass is a copper alloy.

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Why do penguins walk so far to get to their nesting grounds?