I disassembled the multi-port valve yesterday and discovered that one of the "legs" of the spider gasket had come loose. I dried it out using a hairdryer on low, and crazy-glued the leg back in place (only getting my fingers stuck a little).

The jury's out as to whether this will solve the blow-by problem, but I'm optimistic as the leg in question was between the outlet of the pump and the return line.