1: With the cable attached to just one arm, transferring force to the other arm and shoe, the link rod has to be adjusted so that both shoes contact together, otherwise operation will be spongy.
2: Ideally the wheel should be on the bike and an assistant engaged. Disconnect one end of the rod and loosen the locknut. Spin the wheel and stop it with a spanner on each arm at once.
3: With your assistant still holding the spanners, check that the clevis aligns with the hole in the arm. If it’s misaligned (as here), one shoe will be doing most of the work, giving poor braking performance.
4: Adjusting the rod length so that the holes line up should firm up operation, although you may need to repeat the process for final adjustment. The brake may also need to bed in again.