Is the pressure still zero? I thought that it cleared up after you shut it down. If the pressure is still zero, then the blockage needs to be cleared enough to establish at least some flow for either trans fluid or motor flush to work. Trans fluid is much thinner than motor oil and can get into places that motor oil can't but if the pump is completely blocked, then it still will not flow. I would put a mechanical pressure gauge on it and make sure that there really is no pressure. Unless the pump was damaged, it just needs to be cleaned and the filter should be replaced as well.