you are probably correct with a leak. air filter clean? is it an older furnace? I believe they say not to use higher than a merv11(?) filter. when we bought our house, there was no output and then furnace shut off on high limit. checked and did not find a filter; household had oodles of children and two dogs and all carpet. found the evaporator was plugged solid. lucked out, we could pull it out and swing it around to clean without removing the Freon. good luck and hopefully it's an easy fix.